CALIFORNIA ENERGY
COMMISSION
Daylight and Retail Sales
TECHNICAL REPORT
October 2003
P500-03-082-A-5
Gray Davis, Governor
CALIFORNIA
ENERGY
COMMISSION
Prepared By:
Heschong Mahone Group, Inc.
Lisa Heschong, Project Director
Fair Oaks, California
Managed By:
New Buildings Institute
Cathy Higgins, Program Director
White Salmon, Washington
CEC Contract No. 400-99-013
Prepared For:
Donald Aumann,
Contract Manager
Nancy Jenkins,
PIER Buildings Program Manager
Terry Surles,
PIER Program Director
Robert L. Therkelsen
Executive Director
DISCLAIMER
This report was prepared as the result of work sponsored by the
California Energy Commission. It does not necessarily represent
the views of the Energy Commission, its employees or the State
of California. The Energy Commission, the State of California, its
employees, contractors and subcontractors make no warrant,
express or implied, and assume no legal liability for the
information in this report; nor does any party represent that the
uses of this information will not infringe upon privately owned
rights. This report has not been approved or disapproved by the
California Energy Commission nor has the California Energy
Commission passed upon the accuracy or adequacy of the
information in this report.
ACKNOWLEDGEMENTS
This report is a part of the Integrated Energy Systems - Productivity and Buildings Science
program, a Public Interest Energy Research (PIER) program. It is funded by California
ratepayers through California's System Benefit Charges administered by the California
Energy Commission (CEC) under (PIER) contract No. 400-99-013, and managed by the
New Buildings Institute. Heschong Mahone Group would like to acknowledge the support
and contributions of the individuals below:
Heschong Mahone Group, Inc.: Principal in Charge: Lisa Heschong. Project Director: Lisa
Heschong. Project staff: Cynthia Austin, Sean Denniston, Charles Erhlich, Carey Knetch,
Douglas Mahone, Mudit Saxena, Heschong Mahone Group.
Subcontractors: RLW Analytics, Inc: Dr. Roger Wright, RLW Analytics, Inc. statistical
metholodies; Ramona Peet, analyst. Wirtshafter Associates: Dr. Robert Wirtshafter, census
and GIS analysis
Review and Advisory Committee: Dr. Jed Waldman, California Department of Public
Health; Dr. Gage Kingsbury, Northwest Evaluation Association; Dr. Judith Heerwagen,
private consultant; Abby Vogen, Wisconsin Energy Center; Dr. Cliff Federspiel, Center for
the Built Environment; Barbara Erwine, Cascadia Consulting; Dr. Robert Wirtshafter,
Wirtshafter Associates, Inc.
Participant support: This project was supported by many managers and employees of the
participant retail corporation. We are greatly appreciative of their time, facilitation and
review of this study. Studies such as this are highly unusual, in that they require a leap of
faith on the part of corporations that are traditionally highly private and protective of internal
data. We have promised anonymity to this corporation, as we did with the previous study,
and request that everyone involved will endeavor to respect that request and honor the trust
placed in the study team.
PREFACE
The Public Interest Energy Research (PIER) Program supports public interest energy
research and development that will help improve the quality of life in California by
bringing environmentally safe, affordable, and reliable energy services and products to
the marketplace.
This document is one of 33 technical attachments to the final report of a larger research
effort called Integrated Energy Systems: Productivity and Building Science Program
(Program) as part of the PIER Program funded by the California Energy Commission
(Commission) and managed by the New Buildings Institute.
As the name suggests, it is not individual building components, equipment, or materials
that optimize energy efficiency. Instead, energy efficiency is improved through the
integrated design, construction, and operation of building systems. The Integrated
Energy Systems: Productivity and Building Science Program research addressed six
areas:
♦ Productivity and Interior Environments
♦ Integrated Design of Large Commercial HVAC Systems
♦ Integrated Design of Small Commercial HVAC Systems
♦ Integrated Design of Commercial Building Ceiling Systems
♦ Integrated Design of Residential Ducting & Air Flow Systems
♦ Outdoor Lighting Baseline Assessment
The Program’s final report (Commission publication #P500-03-082) and its attachments
are intended to provide a complete record of the objectives, methods, findings and
accomplishments of the Integrated Energy Systems: Productivity and Building Science
Program. The final report and attachments are highly applicable to architects,
designers, contractors, building owners and operators, manufacturers, researchers, and
the energy efficiency community.
This Daylighting and Retail Sales Report (Product # 2.3.7) is a part of the final report
within the Productivity and Interior Environments research area and presents the results
of a study into relationships between daylighting and sales at a retail outlet store.
The Buildings Program Area within the Public Interest Energy Research (PIER)
Program produced these documents as part of a multi-project programmatic contract
(#400-99-413). The Buildings Program includes new and existing buildings in both the
residential and the non-residential sectors. The program seeks to decrease building
energy use through research that will develop or improve energy efficient technologies,
strategies, tools, and building performance evaluation methods.
For other reports produced within this contract or to obtain more information on the
PIER Program, please visit www.energy.ca.gov/pier/buildings or contact the
Commission’s Publications Unit at 916-654-5200. All reports, guidelines and
attachments are also publicly available at www.newbuildings.org/pier.
ABSTRACT
This study presents evidence that a chain retailer is experiencing higher sales in daylit
stores than in similar non-daylit stores. Statistical models, using up to 50 explanatory
variables, examine the relationship between average monthly sales levels and the
presence of daylight in the stores, while simultaneously controlling for more traditional
explanatory variables such as size and age of the store, amount of parking, local
neighborhood demographics, number of competitors, and other store characteristics.
The study included 73 store locations in California, of which 24 stores were daylit
primarily by diffusing skylights. Statistical regression models found that increased
annual hours of useful daylight per store were strongly associated with increased sales,
but at a smaller magnitude than a previous study. No season variation in the
relationship of daylight to sales was found. The study also included interviews with store
managers and surveys of employees, along with an analysis of the energy savings due
to automatic control of the electric lights.
Author: Lisa Heschong, Heschong Mahone Group
Keywords: Daylight, Productivity, Retail, Sales, Stores, Window, Skylight, Design
RETAIL AND DAYLIGHTING TABLE OF CONTENTS
TABLE OF CONTENTS
1. INTRODUCTION ____________________________________________________ 1
2. SELECTION OF STUDY PARTICIPANT _________________________________ 3
2.1. Selection Criteria _________________________________________________ 3
2.2. Participant Search ________________________________________________ 4
2.3. Participant Description _____________________________________________ 5
3. DATA COLLECTION _________________________________________________ 7
3.1. Corporate Databases and Records ___________________________________ 7
3.2. Corporate Interviews ______________________________________________ 7
3.3. On-site Visits ____________________________________________________ 8
3.3.1. Surveyors and Training _______________________________________ 8
3.3.2. Survey Equipment ___________________________________________ 8
3.3.3. Survey Protocol _____________________________________________ 9
3.3.4. Manager Interview __________________________________________ 10
3.3.5. Lighting Quality Survey ______________________________________ 10
3.4. Census Demographic Data ________________________________________ 10
3.5. Local Market Analysis ____________________________________________ 11
3.6. Data Verification _________________________________________________ 12
3.7. Parking Data Verification __________________________________________ 12
4. DATA PROCESSING AND VARIABLE DEFINITION_______________________ 13
4.1. Dependent, or Outcome, Variables __________________________________ 13
4.2. Independent, or Explanatory, Variables: ______________________________ 14
4.2.1. Daylight Variable Definition ___________________________________ 16
4.2.2. Energy Observations ________________________________________ 17
5. STATISTICAL METHODOLOGY ______________________________________ 19
5.1. Variable Testing Method __________________________________________ 19
5.2. Preliminary Investigations _________________________________________ 21
5.2.1. Defining the Core Model _____________________________________ 21
5.2.2. Simple Yes/No Daylight Model ________________________________ 21
5.2.3. Daylight Hours Analysis ______________________________________ 22
5.2.4. Demographic Test __________________________________________ 25
5.3. Final Regression Models __________________________________________ 25
5.3.1. Comparison of Linear versus Log Models ________________________ 26
6. ANALYSIS FINDINGS _______________________________________________ 29
6.1. Log Models_____________________________________________________ 29
6.1.1. Daylight Effect Interaction with Parking __________________________ 31
i
RETAIL AND DAYLIGHTING TABLE OF CONTENTS
6.1.2. Daylight Effect as a Function of Daylight Hours____________________ 32
6.2. Linear Models___________________________________________________ 34
6.3. Discussion of Findings ____________________________________________ 36
6.3.1. Variable R2 and Order of Entry ________________________________ 36
6.3.2. Comparison with Previous Study _______________________________ 36
6.3.3. Comparison of 10 Month and 24 Month Time Periods ______________ 37
6.3.4. Other Findings _____________________________________________ 38
6.4. Additional Analysis _______________________________________________ 40
6.4.1. Seasonal Effects ___________________________________________ 40
6.4.2. Number of Transactions______________________________________ 41
7. OTHER STUDY FINDINGS ___________________________________________ 43
7.1. Energy Impacts _________________________________________________ 43
7.1.1. Store and Corporate Energy Impacts ___________________________ 43
7.1.2. Statewide Energy Impacts ____________________________________ 44
7.1.3. Energy Impacts Relative to Daylight Effect on Sales________________ 45
7.2. Employee Assessment of Lighting Quality _____________________________ 45
7.3. Manager Assessment of Lighting Quality______________________________ 47
8. CONCLUSIONS AND DISCUSSION____________________________________ 49
8.1. Comparison with Previous Retail Study _______________________________ 50
8.1.1. New Analysis Insights _______________________________________ 51
8.1.2. Why Daylight Hours is a Better Variable than Daylight Yes/No ________ 51
8.2. Possible Mechanisms for a Daylighting Effect on Sales __________________ 52
9. APPENDICES _____________________________________________________ 57
9.1. Retail Survey Forms______________________________________________ 57
9.1.1. On-site Survey Form ________________________________________ 58
9.1.2. Manager Interview __________________________________________ 60
9.1.3. Employee Survey___________________________________________ 61
9.1.4. Natural log Models __________________________________________ 63
9.1.5. Linear Models _____________________________________________ 66
9.1.6. Linear Transaction Models____________________________________ 69
9.2. Parking Area Verification Process ___________________________________ 70
9.3. Statistical Terminology ____________________________________________ 71
ii
RETAIL AND DAYLIGHTING TABLE OF CONTENTS
TABLE OF FIGURES
Figure 1: "Core" Model Significant Variables _________________________________ 21
Figure 2: Daylight Yes/No Model, List of Significant Variables ___________________ 22
Figure 3: Daylight Hours as Outcome Variable _______________________________ 23
Figure 4: Diagrammatic Graphs of Linear v Log Scales ________________________ 26
Figure 5: Consistency of Daylight Effects – Linear vs. Log Models ________________ 27
Figure 6: Comparsion of Daylight Effects per Store – Linear and Log Models _______ 27
Figure 7: Range of Daylight Effects per Store predicted by Linear v Log Models _____ 28
Figure 8: Results of Log Models __________________________________________ 30
Figure 9: Graph of Predicted Range of Daylight Effect per Store, Log Models _______ 31
Figure 10: Net Effect of Daylight on Sales, Log Models_________________________ 31
Figure 11: Daylight Effect relative to Parking Area ____________________________ 32
Figure 12: Daylight Effect Independent of Parking, Log 10 Month Sales, 2001_______ 32
Figure 13: Daylight Effect as a Function of Daylight Hours, Log 10 Month Sales, 2001 33
Figure 14: Daylight Effect as a Function of Daylight Hours, Log 24 Month Sales, 1999-
2000 ____________________________________________________________ 33
Figure 15: Results of Linear Sales Model ___________________________________ 34
Figure 16: Net Effects of Daylight on Sales, Linear Models______________________ 35
Figure 17: Graph of Predicted Range of Daylight Effect per Store, Linear Models ____ 35
Figure 19: Net Effect of Daylight on Number of Transactions per Store ____________ 41
Figure 20: Employee Assessment of Lighting Quality __________________________ 46
Figure 21: Hourly Illumination Patterns of Average Daylit and Non-Daylit Stores, 10-
month and 24-month periods _________________________________________ 53
APPENDICES
Figure 22: Summary Statistics for Natural Log Models _________________________ 63
Figure 23: Log Model of 10-Month Sales, 2001 _______________________________ 64
Figure 24: Log Model of 24-Month Sales, 1999-2000 __________________________ 64
Figure 25: Order of Entry and Partial R2, Log 10 Month Sales, 2001 ______________ 64
Figure 26: Order of Entry and Partial R2, Log 24 Month Sales, 1999-2000 __________ 65
Figure 27: Summary Statistics for All Variables Considered in Linear Models _______ 66
Figure 28: Linear Model of 10 Month Sales, 2001 _____________________________ 67
Figure 29: Linear Model of 24 Month Sales, 1999-2000 ________________________ 67
Figure 30: Order of Entry and Partial R2, Linear 10 Month Sales, 2001 ____________ 68
Figure 31: Order of Entry and Partial R2, Linear 24 Month Sales, 1999-2000 ________ 68
Figure 32: Linear Model of 10 Month Transactions, 2001 _______________________ 69
Figure 33: Linear Model of 24 Month Transactions, 1999-2000 __________________ 69
Figure 34: Glossary of Statistical Terminology________________________________ 71
iii
RETAIL AND DAYLIGHTING TABLE OF CONTENTS
iv
RETAIL AND DAYLIGHTING EXECUTIVE SUMMARY
EXECUTIVE SUMMARY
This study presents evidence that a major retailer is experiencing higher sales in daylit
stores than in similar non-daylit stores. Statistical models were used to examine the
relationship between average monthly sales levels and the presence of daylight in the
stores, while simultaneously controlling for more traditional explanatory variables such
as size and age of the store, amount of parking, local neighborhood demographics,
number of competitors, and other store characteristics. The retailer, who will remain
anonymous, allowed us to study 73 store locations in California from 1999 to 2001. Of
these, 24 stores had a significant amount of daylight illumination, provided primarily by
diffusing skylights.
This study was performed as a follow-on to a similar study completed for Pacific Gas
and Electric in 19991, which found that for a certain retail chain, all other things being
equal, stores with skylights experienced 40% higher sales than those without skylights.
This study, on behalf of the California Energy Commission, examined a second retail
chain, in an entirely different retail sector, to see if the original findings would hold in a
new situation, and if we could learn more about any daylight effect that might exist.
As a first step in this process, a simple model with daylight as a yes/no variable, and
using basically the same format and inputs as the previous study, did not find a
significant correlation between the presence of daylight, and increased sales. We then
pursued the study in greater detail, adding more information to the model and describing
daylight on a continuous scale by the number of daylit hours per year in each store.
The retailer in this study had a less aggressive daylighting design strategy and also
more variation in both the range of daylight conditions and the range of store designs
than the retailer in the first study. For this study, we collected much more detailed
information about the characteristics of each store, and verified all information on site.
Neighborhood demographics and retail competition were described using detailed, site-
specific GIS analysis. Store managers were interviewed and employees were surveyed
about their observations and preferences. For the final analysis, the amount of daylight
in each store was described as the number of hours per year that daylight illumination
levels exceeded the design electric illumination level.
Statistical regression models of average sales for the stores, using up to 50 explanatory
variables, and both linear and natural log descriptions of the variables, found that
increased hours of daylight per store were strongly associated with increased sales, but
at a much smaller magnitude than the previous study. In addition, for this chain, the
daylight effect on sales was found to be constrained by the amount of parking available
at the store site. Sites with parking lots smaller than the norm experienced decreased
sales associated with daylight, while stores with average and ample parking experienced
increased sales as both the amount of daylight and parking increased. The statistical
models were also more comprehensive, explaining about 75% of the variation in the
data (model R2=0.75), compared to 58% in the previous study.
1
Heschong Mahone Group (1999). Skylighting and Retail Sales. An investigation into the relationship
between daylight and human performance. Detailed Report for Pacific Gas and Electric Company. Fair
Oaks, CA.
v
RETAIL AND DAYLIGHTING EXECUTIVE SUMMARY
Specifically, this study found that:
• Average effect of daylighting on sales for all daylit stores in this chain was variously
calculated from 0% to 6%, depending on the type of model and time period
considered.
• A dose/response relationship was found, whereby more hours of useful daylight per
year in a store are associated with a greater daylight effect on sales.
• No seasonal patterns to this daylight effect were observed.
• A bound of an empirical daylight effect for this chain was detailed, with a maximum
effect found in the most favorable stores of about a 40% increase in sales. This
upper bound is consistent with our previous finding.
• Daylight was found to have as much explanatory power in predicting sales (as
indicated by the variable’s partial R2) as other more traditional measures of retail
potential, such as parking area, number of local competitors, and neighborhood
demographics.
• Along with an increase in average monthly sales, the daylit stores were also found
to have slightly smaller increase in the number of transactions per month.
• The retailer reported that the primary motivation for the inclusion of daylight was to
save on energy costs by having photocontrols turn off electric lights when sufficient
daylight was detected. The retailer has been very pleased with the resulting
reduction in operating costs. Based on current energy prices we estimated average
whole building energy savings for the daylit stores at $0.24/sf for the current design,
with a potential for up to $0.66/sf with a state-of-the art design.
• The value of the energy savings from the daylighting is far overshadowed by the
value of the predicted increase in sales due to daylighting. By the most conservative
estimate, the profit from increased sales associated with daylight is worth at least 19
times more than the energy savings, and more likely, may be worth 45-100 times
more than the energy savings.
• During the California power crisis of 2001, when almost all retailers in the state were
operating their stores at half lighting power, the stores in this chain with daylight
were found to benefit the most, with an average 5.5% increase in sales relative to
the other non-daylit stores within the chain (even while all stores in this chain
increased their sales compared to the previous period).
• Employees of the daylit stores reported slightly higher satisfaction with the lighting
quality conditions overall than those in the non-daylit stores. Most strikingly, they
perceived the daylit stores to have more uniform lighting than the non-daylit stores,
even though direct measurements showed both horizontal and vertical illuminance
levels in the daylight stores to be substantially less uniform.
• Store managers did not report any increase in maintenance attributable to the
skylights.
• The chain studied was found to be saving about $0.24/sf per year (2003 energy
prices) due to use of photocontrols, which could potentially increase up to $0.66/sf
per year with an optimized daylighting system.
vi
RETAIL AND DAYLIGHTING INTRODUCTION
1. INTRODUCTION
The Skylighting and Retail Sales study1 completed in 1999 by the Heschong Mahone
Group on behalf of the California Board for Energy Efficiency found a compelling
statistical correlation between the presence of daylighting in a chain retail store and
higher sales for those stores.
The study was reviewed by a panel of experts, recruited by Lawrence Berkeley National
Laboratory, involving a wide range of disciplines related to the study. In general, the
review panel was satisfied with the soundness of the basic methodology and the rigor of
the statistical analysis.
There were, however, some weaknesses to the original study and lingering peer review
questions,2 that could only be addressed in follow-up studies.
1. Replicating findings: The biggest weakness in the original study was that
the participant remained anonymous, making it impossible for anyone else to
verify the findings. Anonymity was difficult to overcome, since it was unlikely
that any retailer would be willing to reveal their identity in a study that publicly
discussed sales effects. However, a second study, of another retailer, would
increase confidence that such a skylighting effect could be replicated.
2. Controlling for other influences: The original study controlled for twelve
potential influences on sales. Not all stores in the study were visited to verify
conditions. It was highly probable that there were other factors affecting sales
that were collinear with skylighting that the original research team could not
determine. A more detailed study, including verification visits to all sites in the
study, and collection of more information about store characteristics, should
be able to reduce the uncertainty that other factors collinear with skylighting
might actually be responsible for the original findings.
3. Bounding the effect: The 40% increase in sales associated with skylighting
seemed to be improbably high. At best it could be assumed to be an upper
bound of an effect. If we found positive sales associated with daylighting in
another chain, could we establish upper and/or lower bounds to the effect?
4. Investigating temporal effects or other causal mechanisms: If we found
positive sales associated with daylighting in another retail chain, could we
determine if it had a seasonal nature, associated with longer hours of daylight
in the summer, or a daily effect, associated with more intense levels of
daylight during the middle of the day? Alternatively, might a positive daylight
effect be related to increased customer loyalty, improved employee moral, or
some mechanism less tied to temporal variation in daylight availability?
1
Heschong Mahone Group (1999). Skylighting and Retail Sales. An investigation into the relationship
between daylight and human performance Detailed Report for Pacific Gas and Electric Company. Fair
Oaks, CA.
2
Heschong Mahone Group (1999). Daylighting and Productivity. An investigation into the relationship
between daylight and human performance. Review Report. Fair Oaks, CA.
1
RETAIL AND DAYLIGHTING INTRODUCTION
The study described in this report, supported through the California Energy
Commission's Public Interest Energy Research (PIER) program, was designed to
address these concerns, while also expanding other areas of our knowledge about the
interaction of retail sales and daylighting.
In this study, a second retailer was identified who had appropriate conditions for such a
study, and who was willing to participate in the study. As with the original retail study,
strict anonymity was requested and observed. The retailer provided us with
dimensionless monthly sales index data for each store for a 34-month period. The
research team then identified 73 store sites appropriate for the study, one-third with
daylighting, and collected extensive data about each site.
The research team’s information about each store site was used in a statistical
regression model, and for secondary analysis. This information included:
1. Information about the size, age, history and monthly sales volumes of the
stores (from corporate sources)
2. Population characteristics within a radial distance of the stores (from U.S.
Census 1990 and 2000)
3. Number of competitors within a radial distance of the stores (from public
databases)
4. On-site observations about the neighborhood and about the stores’
architectural features, skylighting system, lighting, mechanical systems, and
other site-specific conditions.
5. Interviews and surveys with store managers and employees.
This data was processed and put into a multivariate regression model. A number of
modeling approaches were investigated. Monthly data allowed us to look for seasonal
patterns. Two different electric lighting conditions during the study period allowed us to
examine illumination intensity issues. Although we were not allowed to interview
customers, interviews with store managers and surveys of store employees allowed us
to examine attitudes and perceptions associated with daylighting. A range of daylighting
conditions within the participant’s store sites allowed us to probe for a dose/response
relationship between daylighting and sales.
Finally, the analysis results were studied, and conclusions were drawn about the role of
daylighting in the sales of this retail chain. This report describes the data and analysis
methodologies in greater detail. Conclusions are then presented in Chapter 8. The
Appendices include the data collection forms and other study details, as well as a brief
glossary of statistical terms to assist readers who are less familiar with the statistical
methods utilized by this study.
2
RETAIL AND DAYLIGHTING SELECTION OF STUDY PARTICIPANT
2. SELECTION OF STUDY PARTICIPANT
Selection of a participant for a large statistical study is a very important and strategic
investment. We invested a considerable amount of time and effort to find the most
promising retail corporation to study for our second effort at understanding how daylight
might effect retail sales patterns.
2.1. Selection Criteria
Prior to beginning the search for a participant retail store, we determined a set of ideal
characteristics that we would use to evaluate and qualify the candidate retail stores. The
following criteria guided our selection process. They were intended to maximize the
potential significance of the study and to minimize confounding factors.
At a minimum, the retail store selected for this study would:
1. Be a large chain retailer with consistent building size, merchandising
practices, merchandise layout and product selection across its stores.
2. Have a large number of stores in relatively small geographical region. The
strength of the statistical analysis is directly related to the number of store
sites studied, so the chain would ideally have at least one hundred sites
available for study. The closer these stores are to each other, the lower the
data collection costs would be for on-site visits, and the more likely that the
stores will have similar climate profiles.
3. Have some daylit and some non-daylit stores so that daylighting effects
could be compared between otherwise identical environments. It would be
ideal to have a continuous range of daylighting conditions so that a dose-
response relationship between daylighting and sales could be studied.
4. Maintain a database on the performance of each store. This information
would most likely be sales data, but could be other metrics of store
performance. The finer the grain of the sales data in terms of time period or
sales department, the more detailed the analysis we would be able to do.
Similarly, if the participant could provide data on the characteristics of the
individual store locations, we would be able to invest project resources in
other types of data collection and thus conduct a more precise analysis
overall.
5. Be willing to participate. The research could not proceed without the
corporation’s permission to utilize their sales tracking data and to allow us to
physically inspect their buildings. Enthusiasm for the study was likely to
facilitate and expedite such access.
6. Be willing to allow the study results to be published publicly. As a project
funded with public goods moneys from the State of California, we are
obligated to make our findings public. If the participant preferred to remain
3
RETAIL AND DAYLIGHTING SELECTION OF STUDY PARTICIPANT
anonymous, we could accommodate this request with careful attention to
confidentiality issues, but the results would be published.
In addition, other desirable, but not required, characteristics included:
7. Allow us to collect data at each store location, if necessary, to complete
our data gathering process. It was unlikely that all of the information
necessary to control for other influences on performance would be available
in the existing data. Therefore, we were likely to need to collect additional
data on site.
8. Have little variation between daylit and non-daylit stores other than the
amount of daylight available. This would minimize the number of other factors
that needed to be controlled for.
9. Be a different retail sector than the original participant. A specific goal of
this project was to study a retail participant in a different market sector than
the previous study, so that we could start to understand the range of
applications where daylighting may have an effect and bound the magnitude
of those effects.
10. Allow us to interview store customers to understand how customer
perceptions and attitudes toward the store relate to the productivity of each
location. In the absence of direct customer interviews, alternative approaches
to collecting customer reactions could be considered.
11. Allow us to interview store personnel. Obtaining the opinions of store
employees could be in addition to, or as an alternative to, interviewing
customers and could help us understand influences on store performance
from a wider perspective.
2.2. Participant Search
The above selection criteria provided us with a basis for deciding the appropriateness of
various candidate retail participants. Our search for participants involved reviewing
library information, examining web-based resources and conducting interviews with
potential candidate stores. Our research identified about a dozen chain stores as
potential study sites, who we then interviewed about their interest in participating in the
study.
Our initial search process resulted in four candidate chain stores that expressed interest
in the study and met our basic criteria. We referred to these four potential participants as
Retailer A, B, C and D. We then interviewed each candidate more closely about the
implications and potential for participation. We particularly focused on the number of
stores with and without skylights, the variety of daylighting conditions and the presence
of any confounding factors, especially the presence of any obvious store characteristics
that might be collinear with the presence of skylights at particular store locations, such
as higher ceilings only in skylit stores, or skylights only in new stores.
4
RETAIL AND DAYLIGHTING SELECTION OF STUDY PARTICIPANT
Each of the four chain stores had very different characteristics of skylight distribution and
store design issues. The corporate history of the chains also varied widely. Two
participants, Retailer A and B, showed the most potential for study.
The team discussed the possibility of doing two studies, each with less depth, in order to
increase the diversity of the study. We received initial site characteristics data from both
candidates and visited a sample of sites from both retailers. From the initial site
reconnaissance, we concluded that Retailer B had some particularly confounding
variables that we would not be able to fully control for in our statistical analysis. Retailer
A seemed to have fewer confounding issues, and more data available about store
performance, history and design characteristics. At this point, Retailer A allowed us to
review store plans, and provided additional information that reassured us about the
feasibility of the study. We finally decided to work with Retailer A on the study and
expressed our appreciation to the others for their interest.
2.3. Participant Description
The selected study participant, hereafter simply called “the retailer,” is a large chain
retailer who initially indicated that between 50 and 100 sites could be made available for
our study. The participant met all of the above minimum selection criteria (numbers 1-6)
and all of the secondary ideal characteristics with two notable exceptions. As in the
previous study, the participant requested anonymity. In addition, while they allowed us
access to store sites and interviews with employees and managers, they requested that
no customers be contacted or interviewed.
Reviewing the corporate files, we identified 73 store sites that met our study criteria.
These were all located in California. Of the sites included in the study, all but two were
single story buildings. Twenty-four of the 73 sites had some form of daylighting,
primarily with diffuse skylights. While there was a fairly standard store plan and skylight
design, there was enough variation in how the daylighting was accomplished among the
daylit stores that we felt we might be able to treat the presence of daylight as a scalar
variable, rather than as a yes/no variable as in the previous study.
5
RETAIL AND DAYLIGHTING SELECTION OF STUDY PARTICIPANT
6
RETAIL AND DAYLIGHTING DATA COLLECTION
3. DATA COLLECTION
Data collection for the study proceeded from a number of sources. First we collected as
much information as possible from the retail participant directly, from corporate records,
plan rooms, and interviews with corporate managers. We then conducted on-site
surveys of every store to be included in the study, to confirm information from the
corporate records and to collect new, detailed data about the physical conditions at each
store. Next we collected and processed Census and market conditions information from
various public databases, using GIS analysis to create site specific information. Finally,
we analyzed interviews with store managers and surveys of store employees to gain a
more qualitative understanding of the conditions at each store.
3.1. Corporate Databases and Records
The retailer maintained databases that included each site’s location, age, size of building
and sales areas in square feet, number of product lines, monthly sales, and number of
sales transactions. They provided us with this data, including 34 months of the monthly
sales data.
The retailer also maintained a reference set of miniature architectural plans, aerial
photos, and other construction and maintenance records. These were examined for
each site to determine the layout of the sales floor, the length of street frontage, the
number of parking spaces, and in most cases, the ceiling height and the lighting system
type and layout. We were also able to review lighting maintenance records to determine
the most recent relamping period for each store and other operational details. Two
surveyors reviewed the plans and filled out a Plan Review Survey Form.
3.2. Corporate Interviews
From telephone and in-person interviews, we gathered information about the history of
particular stores and why some sites had skylights while others did not. This historical
data helped us to determine if there were any factors that might prove collinear between
skylighting and store sales performance.
We learned that the retailer had a wide variety of ownership/tenant relationship for their
store sites. Skylights were typically installed in sites that were acquired for construction
of a new store, regardless of whether the store site was to be owned or leased. Stores
without skylights typically had been acquired from another chain and remodeled to meet
the retailer’s needs. The company felt that it was too expensive to retrofit skylights into
an existing store shell. Occasionally skylights were added to older store sites when
extensive remodels were undertaken.
We also probed for other site variables that the retailer thought were likely to particularly
affect sales performance. This information helped us to decide what additional
information we should try to collect about the sites in order to control for other influences
on sales.
7
RETAIL AND DAYLIGHTING DATA COLLECTION
3.3. On-site Visits
Based on the assessment of available data, we determined that we would need to visit
all 73 study sites, to collect additional information and verify the information provided in
the corporate records.
The retailer gave us parameters of when and how to conduct the surveys to minimize
any intrusion on store operations. We had to limit each site visit to less than one hour,
and minimize the use of instrumentation. We were limited to data that we could reliably
collect within the one-hour site visit. In addition to simple observation, photography and
instrument readings, we would be allowed to interview the store manager and ask the
manager to have employees complete a simple lighting quality survey.
3.3.1. Surveyors and Training
Three surveyors, who were all were permanent employees of the Heschong Mahone
Group (HMG), collected the on-site data. All of the surveyors were architecturally trained
and had a background in daylighting. The surveyors wore neutral colored clothes such
as khakis to minimize influence on the light meter readings.
The surveyors practiced the survey methods together in an initial store that was part of
the chain, but not part of the study. They discussed the interpretations of each field in
the survey data collection form, and practiced finding the standard locations for photos
and instrument readings. In addition to the standard photograph locations, surveyors
were encouraged to take additional photographs to help explain any unique conditions
found at a store. Throughout the on-site survey period, surveyors met periodically to
discuss findings and the survey instrument to aid in the normalization of results.
A generic version of the survey instrument is included in the Appendix, providing more
specifics on the format of on-site data collection.
3.3.2. Survey Equipment
The surveyors used the following equipment:
Clipboard with:
Floor Plan: an 8.5 x 11” Xerox of the store floor plan(s)
Plan Review Survey Form: copy of the plan review survey for each store, both
for reference and verification or completion.
Site and Manager Survey Forms: blank site and manager survey forms
Authorization: Letter of permission to visit site from retailer headquarters
Camera: a Toshiba PDR-M70 digital camera.
Light Meter: a hand held Minolta TL-1 Illuminance Reader. Illumination readings were
taken in footcandles; A 10x filter allowed for outdoor daylight readings. Only one
illuminance meter was used to avoid calibration inconsistencies.
Thermometer: a hand held digital dry bulb and temperature meter for taking dry-bulb
temperature readings, in degrees Celsius.
Anometer: hand held meter for taking air movement readings in ft/min.
Decibel meter: hand held meter for taking ambient noise level readings in dBA.
8
RETAIL AND DAYLIGHTING DATA COLLECTION
Flicker Checker: a spinning tool from Motorola for checking for the presence of
electronic flicker in the lighting.
Tape measure: to measure dimensions not found in the plan review.
3.3.3. Survey Protocol
Upon receiving permission from the store manager to start the survey, the surveyor took
the initial outdoor horizontal illuminance reading and exterior photos at designated
locations. The surveyor then also made other exterior site observations about the
neighborhood conditions, building signage, size of the main street, visibility from the
main street and sky conditions at the time of the survey.
Next, the surveyor confirmed information about the store that had been collected from
the earlier review of corporate records and/or shown on the plan. Interior building
information recorded included surface reflectance observations, luminaire
characteristics, skylight characteristics, thermal environment and acoustic environment.
In addition, illuminance measurements were taken, and several photographs were taken
from standard vantage points to document building conditions.
Four sets of illuminance measurements were taken at the check out area and the
primary, secondary and back aisles of the store in order to quantify the variety of lighting
conditions in the store. At each location, the hand-held measurements included a
horizontal measurement at 4 feet in the center of the aisle (typical shopping cart and
display height), and vertical measurements on the face of the product at 2 feet, 4 feet
and 6 feet on each side of the aisle (heights easily managed by the surveyors without
use of aids). In skylit stores, the aisle measurement sets were doubled, with one set
taken as directly underneath a skylight as possible and one set taken in between two
skylights. The goal was to quantify the maximum range of illumination conditions found
in the store. This procedure was slightly modified from the Lighting Baseline Study of 25
California Retail Stores1. All other readings, such as temperature and noise readings,
were taken at the center of the store.
At the conclusion of the survey, the surveyor took a second reading of exterior
illumination levels. The average of the entrance and exit readings were later used to
normalize the interior daylight readings.
Site visits were scheduled during non-peak sales periods and were completed within 30
to 60 minutes. Visits to skylit stores were preferentially scheduled towards the middle of
the day, between 10 AM and 3 PM, in order to measure full daylight conditions. Non-
skylit stores were often visited earlier or later, or even at night, since we were only
measuring electric illumination. The site visits were completed within an eight-week
period, from late January to early March of 2002.
The fact that not all site visits were conducted during the same time of day or week
made some site observations more suspect. For example, the surveyor observed noise
levels and perceptible air movement, but these observations are likely to be a function of
time of day and the intensity of customer activity at the time of observation.
Upon completion, a copy of all photographs and on-site data collection was provided as
a service to the retailer for their records.
1
Heschong Mahone Group, Lighting Baseline Study, for Southern California Edison, 2000. Presented at the
IESNA conference 2001, Ottawa Canada.
9
RETAIL AND DAYLIGHTING DATA COLLECTION
3.3.4. Manager Interview
The short interview with the store manager offered an opportunity to collect information
about a store that would not be readily apparent from the corporate records or a site
visit. For example, we asked if there had been any disruptions to sales in recent history,
due to nearby construction, natural disasters, power outages, or other intermittent
events. Similarly, the store manager was usually in a position to tell us about the recent
arrival of competitors in the neighborhood or about special attributes of that particular
store or location that we might not otherwise notice.
Store manager interviews were kept confidential and not provided to the retailer.
3.3.5. Lighting Quality Survey
Finally, we developed a lighting quality survey to be administered to the employees of
each store. This survey was modified from a lighting quality survey originally developed
by Dr. Peter Boyce at the Lighting Research Center for office settings. It was
subsequently modified to a retail and school format for the Lighting Baseline Study1
sponsored by Southern California Edison. That study collected baseline lighting quality
data on 25 examples each of existing, newly constructed California office spaces,
classrooms and retail stores. Those studies found that the survey, which asked only yes-
no questions, tended to be somewhat insensitive, with all respondents rating their
lighting above average. Therefore the survey was revised, with responses requested on
a 1-7 scale instead. The question wording remained the same.
The lighting quality survey forms were handed out by the manager to 20 to 30 retail
sales staff at a convenient time. The lighting quality survey forms were returned to HMG
via a self-addressed stamped envelope. We ultimately received an average of 18
responses per store.
3.4. Census Demographic Data
From our previous retail daylighting study, we learned that the ZIP-code level census
data did not predict retail sales particularly well. The two census variables used in the
original study, average household income and total population by zip code location of
the stores, only achieved 95% significance as a predictor of store sales performance,
and together only explained 3% of the variation in the data. Our goal was to use better
demographic predictors of store sales in the current study.
Current practice in the field of real estate location analysis uses US Census data within
either a fixed radius or a calculated drive time from a proposed store location. Drive time
analysis is often considered the best analysis available. Global Information Systems
(GIS) maps that have up-to-date streets and drive times allow a computer to map out the
distances from any location that take, say, 10 minutes to drive at normal traffic speeds.
Such a calculation allows accurate comparisons of residents within an accessible
distance of an urban store site surrounded by slow surface streets and a suburban store
site located off of a fast highway.
1
Heschong Mahone Group, Lighting Baseline Study, for Southern California Edison, 2000. Presented at the
IESNA conference 2001, Ottawa Canada.
10
RETAIL AND DAYLIGHTING DATA COLLECTION
Our goal was to create a reliable comparison between sites in our study as a control
variable, rather than pre-determine the most favorable location for a new store. An initial
comparison of a few store sites in our study showed that the simpler fixed radius
analysis captured population effects within 85 to 90% accuracy of the far more complex
drive-time analysis. In addition, we noted that many of our store sites were located in
rapidly developing areas where street information was not up-to-date in the GIS
databases. Thus, we determined that using a fixed radius Census analysis would give us
sufficient accuracy, and perhaps also a more reliable comparison between our sites. We
interviewed the real estate manager for the chain and determined the appropriate radius
to use in the census analysis. This is henceforth called the “standard radius.”
We reviewed 34 possible census characteristics with the real estate manager for the
retailer, and together selected twelve characteristics that represented a range of
population, economic, ethnic, housing and transportation information, and that were
considered most relevant to this particular chain’s target customer. We will not identify
the specifics of the census variables considered in order to protect the retailer’s identity.
A GIS consultant processed this information into ten census variables for each study
site. Since each variable was based on census data within the area determined by a
standard radius, the census variables also became density indicators for each site.
At the time of our data collection, the 2000 US Census data was just becoming
available. Population and ethnic characteristic data was available for the 2000 census,
but for housing, economic and transportation data we had to use 1990 data. The
difference between the 1990 and 2000 population data determined growth rates for the
sites.
3.5. Local Market Analysis
We also used a GIS mapping database to locate competitors close to the subject store
sites. The retailer told us whom they considered to be their major competitors. We
determined the number of these competitor stores within one standard radius and twice
the standard radius of each site. We used a simple count of store locations within a fixed
radius, rather than a more sophisticated “gravity” analysis, which attempts to account for
floor area and volume of other competitors relative to distance from a given location.
Since competitor stores tended to be fairly standardized, this provided acceptable
accuracy. This information formed two additional variables considered in the analysis:
Compet 1 (number of competitors within one standard radius) and Compet 2 (number of
competitors within two standard radii).
In addition, co-tenants for any site were observed during the site visit and assigned a
scalar of 0-4 based on the store type, size and typical intensity of customers use. A zero
indicated no co-tenants, one indicated small local stores, while a four indicated an
extremely large (big-box) co-tenant with a steady stream of customers.
Interviews with store managers revealed if there had been any event in the
neighborhood that might have dramatically impacted sales during the time period of the
study. This included such things as major construction nearby which interfered with
customers’ access, or a nearby fire or other disaster that affected sales. This information
was converted to a flag variable that indicated a negative sales event for that store.
The retailer also told us that they had observed an effect whereby additional stores of
the retailer in a given area tended to boost sales for all stores in that area. This was
attributed to the advantages of co-advertising with a given media market and additional
11
RETAIL AND DAYLIGHTING DATA COLLECTION
local customer awareness of the stores. To account for this effect, we create a “sister
store” scalar. We mapped out the stores in the study and counted the number of stores
sharing a similar media market. The store locations were rated, on a scale of 1 to 5, for
the density of other sister stores nearby from the same chain. A store with a rating of 1
was alone in its media market, while a store with a rating of five had the highest density
of sister stores nearby.
3.6. Data Verification
The data from the site visits was collected on paper survey forms, then entered into
electronic databases, with standard error bounds testing and validation features. The
data was checked and processed within Microsoft Access, and then transferred into SAS
for statistical analysis.
All of the site data was examined to make sure that it was reliable and provided a
sufficient range of conditions for useful analysis. The acoustic, dry bulb air temperature
and air movement instrument readings were found to be inconsistent, and frequently out
of bounds, and so were dropped from the analysis. Surveyor observations about noise
sources and perceptible air movement were found to be more believable and consistent
and were used in their place.
3.7. Parking Data Verification
During the course of analysis it was discovered that some of the parking lot counts
collected in the initial plan review phase of data collection did not seem plausible. Many
of the site plans reviewed were old or incomplete, and it was possible that the parking lot
had been modified since the plan date. Since the parking lot variable was quite
significant in initial models of sales performance, we decided to verify the parking lot
counts during the study period.
We obtained parking lot counts from the retailer for about 80% of the store sites.
However, these counts were of uncertain dates and based on a variety of counting
methodologies. We also obtained low-resolution aerial photographs for about 80% of the
sites (not the same 80%), from which we could estimate the parking capacity of the lots.
While the aerial photos were considered the most reliable in terms of time period (they
were all from approximately the study period) they were often difficult to interpret.
After consideration of a number of methodologies, we created a method to select
between the available information sources for a given store. This method is described in
Appendix 9.3. This process resulted in about 25% of the parking counts being revised.
As a result of this process the parking data was brought into a more normal range. This
data was re-entered into the models, and forms the basis of our final reports.
12
RETAIL AND DAYLIGHTING DATA PROCESSING AND VARIABLE DEFINITION
4. DATA PROCESSING AND VARIABLE DEFINITION
Upon completion of data collection and verification, the data was processed into useful
variables for analysis. If a store characteristic did not exhibit sufficient variation between
stores, we could not use it in the analysis. For example, the variety of signage was not
found to vary much between survey sites, and so signage type was not considered in the
analysis. Likewise, whenever fewer than four stores exhibited a particular characteristic,
that characteristic was dropped from the analysis.
In some cases data was combined in order to increase the range of variation in the data
for analysis. For example, the acoustic properties of the stores were originally collected
according to five different properties, but they were subsequently combined into one
acoustic scalar indicating overall noise levels in the stores. Similarly, we were given data
on the square footage and number of products sold for a variety of sales areas. We first
collapsed this information into three types of sales areas, and eventually collapsed it into
a variable named “total sales area scalar.”
Forty-one explanatory variables and two dependent variables were ultimately defined
and included in the preliminary analysis. These variables took the form of binary
variables (yes/no) or scalar variables (a range of values indicating relationships from
small to large). In order to preserve the anonymity of the participant, not all information
about the variable definitions or ranges can be revealed. For reporting purposes, most
variables were transformed into a dimensionless scalar in order to mask identifying
information about the retailer.
Descriptive statistics for the variables considered in the analysis are included in the
Appendix. These include the minimum, maximum, range, mean and standard deviation
for that variable. When a scalar variable is used, the minimum is a dimensionless unit of
one, and the maximum illustrates the relative range of that variable.
4.1. Dependent, or Outcome, Variables
The retailer provided us with 34 months of monthly sales totals and number of
transactions per store site. All these data were transformed into dimensionless scalars
that would not reveal actual amounts, but that could be used consistently in statistical
analysis, with different multipliers used for each type of data.
The 34 month study period included the California “power crisis” of 2001, when most
retailers in California agreed to operate their stores at one-half of normal electric light
levels in order to reduce peak loads on the state electric grid. This voluntary reduction in
light levels, by both retailers and other companies, had an enormous impact in helping to
reduce the peak power demands in California that year, thereby helping to avert many
potential rolling blackouts.
During normal operations our participant had used automatic photocontrols to reduce
electric illumination when sufficient daylight was available in daylit stores, while non-
daylit stores were operated at full light output at all times. During the 10 months of the
power crisis, all stores were operated at reduced illumination levels. Thus, the automatic
photocontrols were overridden and both daylit and non-day lit stores were at
approximately one-half normal electric illumination levels at all times.
13
RETAIL AND DAYLIGHTING DATA PROCESSING AND VARIABLE DEFINITION
We took advantage of this change in operation to create a natural experiment. We
divided our data into two periods: a 24-month period of normal lighting system operation,
during 1999-2000, and a 10-month period when all stores were operated at about one-
half of normal illumination, during 2001.
For each of these two time periods, we analyzed the data with two mathematical
approaches, using both linear and log models of the sales data. The transaction data
were similarly broken into the two periods, but were only analyzed with linear models.
Each outcome variable was considered in a separate regression model.
Outcome variables considered:
• Sales24: Sales index per store, the average of the monthly sales index for the 24-
month period during 1999 and 2000
• Sales10: Sales index per store, the average of the monthly sales index for the 10-
month period during 2001
• Log Sales24: Natural log of the sales index per store, the average of the monthly
sales index for the 24-month period during 1999 and 2000
• Log Sales10: Natural log of the sales index per store, the average of the monthly
sales index for the 10-month period during 2001
• Trans24: Transaction index per store, the average of the monthly transaction index
for 24-month period during 1999 and 2000
• Trans10: Transaction index per store, the average of the monthly transaction index
for 10-month period during 2001
4.2. Independent, or Explanatory, Variables:
Independent variables were considered in five basic groups: corporate level variables,
census variables, local market influences, comfort conditions, and interaction variables.
Below we describe each explanatory variable considered in the analysis and give the
data source. The term “scalar” is applied to variables that have been transformed from
the raw data into a dimensionless scale in order to mask information about the identity of
the retailer Indented variables are variants of the one above, used in preliminary
investigation or final log models. Summary statistics for all variables are described in
Figure 21 and Figure 26 in the Appendix.
CORPORATE LEVEL DATA:
• Area: Total sales area scalar, the relative size of the sales area in each store, per
corporate records
• In preliminary analysis Area was broken into three sub areas, termed Sales Area
1, 2 and 3.
• LogArea; Natural log of the total sales area scalar, used in log models
• Hours: Longer work week yes/no, indicator for a store with hours open longer than
standard, per corporate records
• Age: Store age scalar, relative age of the store, per corporate records
• LogAge; Natural log of the store age scalar, used in log models
14
RETAIL AND DAYLIGHTING DATA PROCESSING AND VARIABLE DEFINITION
• Mgr: Manager seniority scalar, relative seniority in corporation, reported by store
manager
CENSUS DATA (all per standard radius from store location; census year indicated):
• Housing: Housing status, 2000
• Pop: Population density, 2000
• PopGrow: Population growth percentage, (2000-1990)
• Ethnic: Ethnic status, 2000
• Household: Household status, 2000
• Income: Income status, 1990
• Econ: Economic status, 1990
• Education: Education status, 1990
• Language: Language status, 1990
• Transport: Transportation status, 1990
LOCAL MARKET INFLUENCES (source indicated):
• Co-mktg: Number of sister stores within standard radius, GIS analysis
• Compet 1: Number of competitor stores within standard radius, GIS analysis
• Compet 2: Number of competitor stores within twice standard radius, GIS analysis
• Cotenant: Co-tenant scalar, a scale of 0-4 for co-tenants, based on estimated
intensity of customer visits to co-tenant, observed by surveyor
• Lanes: Number of lanes on the main street, observed by surveyor
• Visible: Street visibility scalar, relative visibility of store from primary frontage street,
on a scale of 1-4, observed by surveyor
• Sign: Building signage yes/no, signage for store is typical or atypical, observed by
surveyor
• Event: A negative sales event in neighborhood, yes/no, reported by store manager
• Length: Storefront length scalar, relative length of storefront visible to frontage
street, taken from plans
• Height: Storefront height scalar, relative height of highest part of store frontage,
taken from plans
• Parking: Parking scalar, relative number of parking spaces, taken from plans,
corporate data and aerial photos (see Appendix for data source selection method)
STORE COMFORT CONDITIONS
• DayHrs: Hours of daylight above a certain illumination threshold, as derived from
annual SkyCalc or DOE-2 simulations based on store design and climate location
(discussed in next section)
• Daylight, yes/no (significant daylight in store, other than from entrance façade
glass), used in preliminary investigations
• Daylight, partial area illuminated, yes/no, used in preliminary investigations
• Daylight, from vertical glazing, yes/no, used in preliminary investigations
15
RETAIL AND DAYLIGHTING DATA PROCESSING AND VARIABLE DEFINITION
• VertAvg: Average of all vertical illuminance readings, a measure of intensity of
illuminance (normalized for outside illuminance at time of measurement in daylit
stores), per site measurements
• VertSD: Standard deviation of all vertical illuminance readings, a measure of
uniformity of vertical illuminance levels, per site measurements
• Luminaire: Atypical luminaire yes/no, standard luminaire layout for retailer or
atypical, observed by surveyor
• Lamps: Type of lamps, standard or atypical, used in preliminary investigations
• Lightson: Electric lighting scalar, relative scalar of portion of electric lights on during
study period, based on corporate records and on-site observations
• Ceiling: Ceiling height scalar, relative average height of ceiling, taken from plans
• Air: Noticeable air movement yes/no, observed by surveyor
• Smell: Odor scalar, relative presence of pleasant or unpleasant smells in store,
observed by surveyor
• Noise: Noise scalar, relative distracting noise levels in store, observed by surveyor
• Clean: Cleanliness of store scalar, observed by surveyor
INTERACTION VARIABLES (interaction variables with daylight hours were tested for all
variables that were significant in preliminary models)
• AreaDH: Sales area scalar times daylight hours
• AgeDH: Store age scalar times daylight hours
• PopGrowDH: Population Growth times daylight hours
• MktgDH: Number of sister stores times daylight hours
• Comp1DH: Number of competitors within radius 1 times daylight hours
• HeightDG: Store maximum height scalar times daylight hours
• FrongtageDH: Store length scalar times daylight hours
• ParkDH: Parking area scalar times daylight hours
• AreaDHhours: Store area scalar times daylight hours times longer work week
yes/no
4.2.1. Daylight Variable Definition
In the previous retail study we were only able to describe the presence of daylighting as
a yes/no variable. We were assured in that study that the skylighting design was highly
standardized in all stores, which seemed to be confirmed by site visits to a sample of
sites. Thus, a yes/no variable seemed a reasonably accurate description of conditions
for these stores. However, in this newer study, we hoped to use a more sensitive metric
to describe the amount of daylight in the stores. The new participant had a greater
variety of daylighting conditions, including differences in the type, amount and placement
of skylights, and also included a few stores daylit from roof monitors or clerestories.
We decided to use the number of daylit hours above a certain threshold illumination as
the daylight metric. Threshold illumination was defined as the design horizontal
illumination in non-daylit stores reported to us by the store management (which was also
empirically found to be very close to the observed average horizontal illumination in non-
daylit stores). This daylight hours variable could capture the variation in both intensity
16
RETAIL AND DAYLIGHTING DATA PROCESSING AND VARIABLE DEFINITION
and duration of daylight due to climate location, daylight system and store interior
design. When only a sub-area of the sales floor had useful daylight, the daylit hours
were calculated for that sub-area, then proportioned relative to the size of the store.
Thus, if only one half of the sales area was daylit, the annual daylight hours were
reduced by half.
Number of daylit hours per year per store was predicted by running computerized hourly
simulations of each store, based on building design variables, local climate using typical
meteorological year data (TMY2), type of glazing, amount of glazing area, dimensions
and surface reflectances within the store. This was fairly easy for the standard skylit
stores, using our automated spreadsheet SkyCalc. It was more difficult for the few
stores using non-standard daylighting systems, such as clerestory windows or roof
monitors. For those, we used an annual DOE-2 model, which could account for the
effects of vertical glazing.1
The SkyCalc daylight hour calculations are limited by the granularity of TMY weather
data available for each site that could be used to generate input. There are 16 climate
files available for SkyCalc in California, based on the 16 climate zones defined by the
California Energy Commission for the Title 24 Building Energy Standards. Thus, the
daylight availability analysis is for typical weather in a nearby city representing the
appropriate California climate zone for each site, rather than actual yearly weather for a
specific city or store site.
4.2.2. Energy Observations
Using this method to estimate store daylight system performance, we found that daylight
availability above threshold conditions varied from a low of 270 hours per year, to a high
of 1800 hours per year, with a mean of 1090 and a standard deviation of 409. There are
a total of 4,380 daylight hours available per year (12 hrs * 365 days). Thus, this retailer
was estimated to be reducing electric lighting in the daylit stores about 25% of the
daylight hours.
We were not able to monitor actual practice. Simulation of these skylight systems
suggested that these stores were far below optimum daylight performance. More
aggressive daylighting design could have produced more hours of useful illumination,
and more aggressive photocontrol operation (at a lower threshold) could also have
produced far greater energy savings. A more optimized system could probably have
reduced electric lighting in the daylit stores for about 75% of the daylit hours.
Further discussion of the energy impacts of the design and comparison to the daylight
effect on sales is included in Section 7.1 Energy Impacts.
1
For more information on SkyCalc, see: Heschong, Lisa and Jon McHugh, "Skylights: Calculating
Illumination Levels and Energy Impacts," Journal of the Illumination Engineering Society, Winter 2000,
Vol. 29, No. 1, pp. 90-100, and Skylighting Guidelines, 1999, a web-based publication on skylighting
design, downloadable from www.energydesignresources.com.
17
RETAIL AND DAYLIGHTING DATA PROCESSING AND VARIABLE DEFINITION
18
RETAIL AND DAYLIGHTING STATISTICAL METHODOLOGY
5. STATISTICAL METHODOLOGY
The heart of this study was the statistical analysis of the data collected. This analysis
entailed developing statistical models that seek to explain the factors that affect retail
sales in this particular chain. Our goal was to control for other influences on sales in
order to isolate the effect of our key variable of interest: daylighting. Developing these
models requires both science and insight. It requires reasonable experience with what is
likely to influence sales, a thorough understanding of how reliable the available data is,
and a certain amount of trial-and-error looking for mathematical models which best fit the
data. A variety of statistical tests are used to determine which modeling approach
provides the most mathematically accurate representation of the data. It is important to
remember that the statistical models are a mathematical abstraction of reality. They do
not so much provide true or false answers, as provide a way to simplify a very complex
retail environment and start to quantify the relative magnitude and certainty of various
influences on sales performance.
Regression models try to fit lines that best describe a plot of data points. Multivariate
models consider more than one dimension at once. Linear models try to fit straight lines
through the data. It is also possible, but far more complex, to consider curved, or non-
linear, relationships, as we did with models using a natural log function.
All of the analysis was pursued using multivariate regression models run in SAS using a
variant of backwards step-wise regression to eliminate the least significant variables. F-
tests1 were performed on groups of variables to insure that they could be dropped as a
group as well as individually. The analysis used p≤0.10 as the threshold criteria for
inclusion of explanatory variables in the models, meaning that for a variable to be
considered significant in determining sales, there must be no greater than a 10% chance
of error in making this decision, or 90% certainty. All statistical terms are explained in
Section 9.2 in the Appendix.
Models were judged based on their R2 (the percentage of variation in the data explained
by the model), the parsimony (minimum explanatory variables for maximum explanatory
power), and consistency between the models. Ultimately, models predicting more
moderate effects for daylight were also judged to be more realistic than those with wildly
diverging values.
5.1. Variable Testing Method
There are 3 stepwise variable selection procedures that are often employed in linear
regression: forward selection, stepwise selection, and backward elimination. The forward
selection procedure starts with an equation that contains only the constant term and
successively adds explanatory variables one-by-one, until the last variable added to the
model is insignificant. Stepwise selection is essentially a forward stepwise procedure,
with the exception that at each iteration, the possibility of deleting a variable is also
considered.
1
See Appendix Section 9.2 for an explanation of “F-test” and other statistical terms used in this report.
19
RETAIL AND DAYLIGHTING STATISTICAL METHODOLOGY
The backward elimination method first calls for fitting a model using all potential
explanatory variables and calculating the t-statistic associated with each variable. The
explanatory variables are then deleted from the model one-by-one, until all variables
remaining in the model are associated with a significant t-statistic. During each iteration,
the variable with the least explanatory power is identified and deleted from the model.
The RLW variable selection method1, used in this study, is a variant of the backward
elimination method. Similar to the backward elimination method, the RLW variable
selection method begins with calculating a model using all potential explanatory
variables and the associated t-statistics. However, the RLW method allows for the
deletion of multiple variables during each iteration, whereas the backward elimination
method does not. This procedure helps to identify co-linearities between insignificant
variables, which might otherwise be dropped without first understanding how such co-
linearities could potentially influence results. Specifically, the RLW method consists of
the following steps:
1. Calculate a “full” linear regression model including all potential explanatory
variables.
2. Identify all insignificant variables from the model resulting from step 1.
3. Perform an F-test to test whether the set of individually insignificant variables are
statistically significant as a group. Specifically, the null hypothesis of the F-test is
that the beta coefficients of each of the variables in the group are zero, while the
alternative hypothesis is that there is at least one variable in the group where the
beta coefficient is not zero. If the F-test shows the set of variables are not
statistically significant as a group, all variables identified in step 2 are also
identified for deletion. If the set of variables tested is statistically significant as a
group, this indicates there is a collinear relationship between the variables that is
affecting the model. In this case, a reduced set of variables is defined for the F-
test and deletion from the model.
4. Calculate a reduced model including all explanatory variables that were not
identified for deletion.
5. If any previously significant variables become insignificant in the reduced model,
calculate an F-test for all variables previously deleted from the model and the
newly insignificant variables under the guidelines provided in step 3.
1
The RLW variable selection methodology was developed by Dr. Roger Wright, lead statistician of this
study.
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RETAIL AND DAYLIGHTING STATISTICAL METHODOLOGY
5.2. Preliminary Investigations
We began the statistical analysis with a number of preliminary investigations to help us
identify appropriate variables and the best form of the model.
5.2.1. Defining the Core Model
We had a very long list of potential variables that we wanted to consider as explanatory
variables in this study. To simply this process of identifying the most significant
variables, we began by running simple models, first testing just the corporate level
information, then adding the demographic and marketing variables in groups.
We ran a series of preliminary models testing these variables for consistency between
both the 10-month and the 24-month models. After a series of about four comparative
runs, we settled on a core model with the highest R2 and the most consistent set of
explanatory variables. Figure 1 lists the variables significant in these core models, and
their respective p-values.
“Core” Model, Significant Variables (p≤0.10)
10 month 24 month
Sales Area 1 .018 .032
Sales Area 3 .036 .034
Longer Hours .011 .003
Store Age .000 .000
Population Growth .008 .038
Population Density .078 .053
Household Status .085 .043
Sister Stores .036 .013
Competitors, Radius 1 .016 .006
Height of Storefront .053
Parking Scalar .000 .000
Model R2 68.5 70.2
Figure 1: "Core" Model Significant Variables
The models were tested for “outliers,” or store sites that were performing significantly
different from all the others, and therefore unduly influencing the findings. One store
tested as an outlier and so was isolated from the equation. This store was a very high
selling daylit store.
These two core models were then used to test various ways of defining the daylight
variable and other physical conditions of the individual stores.
5.2.2. Simple Yes/No Daylight Model
First we attempted to replicate the simple models that had been used in the previous
Retail and Daylight study. For this model, daylight was defined as a simple yes/no
variable. In the original study, we had used zip code-based census information. Here, we
used the more detailed, and presumably more accurate, census information by radius.
We also used information about the market conditions of each store. We did not,
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RETAIL AND DAYLIGHTING STATISTICAL METHODOLOGY
however, include physical comfort characteristics about each store. Figure 2 shows the
p-values for significant variables in this model.
Daylight Yes/No Model, Significant Variables (p≤0.10)
10 month 24 month
Sales Area 1 0.018 0.069
Sales Area 2 0.036 0.019
Longer Hours 0.011 0.051
Store Age 0.000 0.001
Population Growth 0.008
Population Density 0.078
Housing 0.085
Education 0.029
Transportation 0.037
# Sister Stores 0.036 0.019
Competitors, Radius 1 0.016 0.011
Frontage Height 0.027
Parking 0.000 0.000
Model R2 68.5 68.7
Figure 2: Daylight Yes/No Model, List of Significant Variables
In these initial models, the yes/no daylight variable was not significant. The R2 of the
models was higher than the previous study (R2 went from 58% to 69%), suggesting the
other variables we included were increasing our precision in predicting sales. The new
census variables were significant, but were not consistent for both models. Likewise, the
height of the storefront was significant in one model, but not both.
5.2.3. Daylight Hours Analysis
Our next set of investigations focused on creating a more precise way to model daylight,
rather than using a simple yes/no indicator. The amount of daylight in a store varies in
intensity through out the day and year. For simple skylit spaces, there is a fairly
predictable relationship that the more intense the daylight is under peak conditions, the
more overall hours of useful daylight there will be per year. Thus, as a measure of both
intensity and duration, we chose to calculate the number of hours per year that the
daylighting illuminance would exceed a certain threshold illuminance. We calculated this
value for various illuminance thresholds. Ultimately, we used the target electric
illuminance for the non-daylit stores as our threshold, as this variable provided the
greatest discrimination in values across the daylit stores. This target illumination level
was obtained from the management, and confirmed by measuring the average
horizontal illuminance for non-daylit stores.
We discovered early in our analysis that many of the variables we defined were highly
correlated with each other. Some of these had a fairly obvious causal explanation, such
as higher ceiling heights and higher average vertical illumination levels in the daylit
stores.
Others sets of correlated variables had no obvious explanation, such as the observation
that daylit stores tended to have slightly larger parking lots. In order to account for all
potential correlations between daylight and other variables, we undertook two tasks.
First we ran a test model with the daylight hours as the outcome variable, as described
below, which highlighted those variables most strongly correlated with daylight. Second,
we identified a set of interaction variables for inclusion in the final models, which
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RETAIL AND DAYLIGHTING STATISTICAL METHODOLOGY
accounted for interaction effect between the presence of daylight and other significant
variables in predicting the sales index.
Identifying Variables Co-Linear with Daylight
In order to understand the magnitude of the collinearities between the daylighting
variable and the other explanatory variables under consideration, we calculated a linear
regression model where the outcome, or dependent, variable was the number of daylight
hours. We allowed all of the information we had about the stores to compete in the
models we tested, and we found that these models explained 70% of the variation in
daylight hours predicted per store. Variables significant above the 90% level (p ≤ 0.10)
included:
Significant Variables Predicting Daylight Hours
(p≤0.10)
Sales Area 2 0.058
Competitors, Radius 2 0.022
Length of store front 0.034
Average of all vertical illuminance 0.000
Electric lighting scalar 0.000
Type of lamps 0.034
Cleanliness of store 0.062
Ceiling type 0.000
Model R2 70.5
Figure 3: Daylight Hours as Outcome Variable
The good news from this exercise was that none of variables that had shown up as
predicting sales in the “core” models were significant in predicting the daylight hours,
thus they were determined not to be collinear with daylighting.
The bad news from this exercise was that there were clearly many potential explanatory
variables that were collinear with daylight hours. In addition to those shown above, a
quick test told us that there were many other variables that were collinear with daylight
hours, but below the model threshold 90% significance level. All of these collinear
variables could potentially cause problems in our subsequent sales analysis,
confounding the effect of the daylight variable on the sales index. This was likely to be
the greatest concern for explanatory variables that were also significant predictors of
sales.
A number of these collinear variables had a logical relationship to the presence of
daylight, such as the ceiling type, which was almost a yes/no indicator for daylight, or
higher vertical illuminance levels, which was almost universally higher in daylight stores.
When there were such obvious dependencies between variables related to daylight
levels, we ran the variables in separate sales index models, testing which of the
competing variables had more significance and predictive power. In these tests, the
daylight hours variable stayed in the model as significant and the other descriptors
dropped out as not significant. Thus, we concluded that daylight hours, rather than other
correlated conditions, were more useful predictors of sales. Therefore, we left the other
variables out of our subsequent models.
While we can never be certain that excluded collinear variables are not influencing the
daylight hour results, we have higher confidence in the daylight hour variable for two
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RETAIL AND DAYLIGHTING STATISTICAL METHODOLOGY
reasons. First, per above, it provided more precision in predicting a sales effect.
Secondly, there was a more obvious hypothesis for a causal relationship.
For similar reasons, at this point we also dropped the electric lighting scalar, type of
lamps, and cleanliness of store variables. We did not have confidence these variables
were reliable. The electric lighting scalar was based on observations at the time of the
survey, but the lighting could easily have been different during the other times of the
study period. The type of lamps variable was based on conflicting evidence from a
number of sources, and may also have changed during the study period. The
cleanliness variable had a very limited range of conditions between stores, and was also
rather subjective. We have seen in our previous study that shoppers tended to associate
daylighting with a cleaner store, and it seemed possible that our surveyors may have
made the same assessment.
Daylight Interaction Variables
To control for the effects of the co-linearities observed above, we created a model that
considered interaction variables between the daylighting variable and all explanatory
variables that had been retained as significant throughout the preliminary tests of
models.
We were especially concerned that Sales Area 2 was collinear with skylighting, even
through it was not showing up in the core models as a significant predictor of sales. Two
versions of the interaction model were tested – one where the different sales areas and
associated interactions were considered separately and one that considered the
aggregate of the various sales areas (total sales area) and an interaction variable
between it and daylighting. The models that considered the different sales areas
separately were generally somewhat illogical, inconsistent and un-interpretable, while
the variant using the total square footage was more stable and easily interpreted. Both
versions had almost identical explanatory power. For this reason, we selected the
variant of the model that was based on the total sales areas.
The use of interaction variables made for more precise models, but also made them a bit
more difficult to interpret directly. With interaction variables, the effect of more daylight in
the stores can only be understood relative to the other influences on the daylight effect.
Interaction variables basically describe second-order effects, which modify the primary
effects of the two variables considered. When using interaction variables, if one
interaction variable is found to be significant, then all of its component parts are also
forced into the model, whether they are significant or not, so that the net effect can be
properly calculated. In the case of one of our models, the 10-month linear sales model,
two daylight variables were kept in the model, even though they fell below the threshold
significance level of p≤0.10.
It is important to recognize that the models using Daylight Hours with interaction effects
are far more complex than the Daylight Yes/No models used in the previous study. The
simple Yes/No Daylight models predicted the same daylight effect for every daylit store.
The Daylight Hours models with their interaction variables, on the other hand, predict a
varying range of effects, per individual store, as a function of each store’s unique
combination of physical conditions. The predicted daylight effect per store is calculated
by applying the model’s equation to each store’s specific characteristics relative to its
daylight hours and interaction variables. Thus, this calculation includes the effects of
total daylight hours as predicted by local climate conditions, store surface reflectances,
24
RETAIL AND DAYLIGHTING STATISTICAL METHODOLOGY
skylight area, etc, and the daylight interaction variables included in that model, such as
parking area or age of the store.
This is a much more nuanced approach to studying the effects of daylight. Sometimes
the daylight effect for an individual store is predicted to be positive and sometimes it is
negative. The key issue of interest is whether the net effects of daylight across the fleet
of stores in the chain is positive or negative. Using the interaction variables, we
calculated the predicted sales for each store according to the models, and then
summarized the net effect on the chain. When we calculated this net effect for the chain
for each model, we found a net positive effect for three of the four models. These values
are reported in the Findings, Section 6.1.
5.2.4. Demographic Test
Somewhat surprisingly, very few, and often only one, of the 10 demographic variables
were found to predict sales in these preliminary models. We hypothesized that two of the
other explanatory variables, which measured the amount of competition within the area,
were absorbing all of the market effects that would be normally predicted by
demographic information.
For example, if both the study retailer, and all of its competitors, were carefully and
consistently analyzing demographic information in order to select new store sites, then
the number of sister and competitor stores within a certain radius would already predict
most of the demographic variables. To test this theory, we ran another series of models,
dropping the sister store and competitor store explanatory variables. When we did this,
two more demographic variables, which characterized the local population’s economic
status, did indeed enter the models at high significance. The R2 of these models,
however, were slightly lower, convincing us that our two “competition” variables did
indeed do a better job of capturing demographic effects.
5.3. Final Regression Models
The mathematics of the regression models can take different forms, depending on the
kind of effect one is trying to study. In many studies, linear regression models are
perfectly adequate, and this is the type of model that was used in our previous
daylighting and retail sales study. For this study, however, we tested two types of
models, one using the linear sales index and the other using the natural log1 of the sales
index.
Log variables have often been found to be highly appropriate for models dealing in
economic functions, or variables likely to have diminishing effects as their size
increases. Since our models were dealing with sales indices, and were also likely to
include diminishing effects, this seemed appropriate.
Using the natural log of the outcome variable basically puts the Y-axis on a log scale
with diminishing effects as one moves up the scale. This is illustrated in the
diagrammatic graphs shown in Figure 4, which plots the same data on the two different
scales. Figure 4 shows that for a log model, a unit change in X value at the low end of
the range makes a bigger difference in Y, than the same change in X value at the high
1
a logarithmic function based on the natural number “e”
25
RETAIL AND DAYLIGHTING STATISTICAL METHODOLOGY
end of the scale. A log model thus makes sense when one expects there to be a case of
diminishing returns, where a unit increase in an explanatory variable at the bottom of the
scale is expected to have a proportionately bigger effect than at the top. For example,
one might hypothesize that doubling the size of a parking lot will have a relatively greater
effect for a small 50 space lot than a large 500 space lot.
Linear Sales Graph Log Sales Graph
450 1000
400
Log of Sales Index
350
Sales Index
300 100
250
200
150 10
100
50
0 1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Figure 4: Diagrammatic Graphs of Linear v Log Scales
We tested models using natural logs of both the dependent variable (sales) and of some
of the explanatory variables that seemed appropriate for a logarithmic scale. A
logarithmic function requires that the variable be defined on a continuous scale, and also
that it does not include any values less than or equal to zero, since the natural logarithm
function is only defined for positive numbers. Thus, only some variables can be
converted to natural logarithms. In addition to meeting the mathematical criteria for
taking the log of a variable, a logged variable should also have a logical explanation for
why a diminishing effect might be expected as the scale of the variable increases.
Using these criteria, we took the natural log of the following explanatory variables and
included them in a “log” model of the Sales Index: Sales Area, Store Age, and Parking.
When we did this, the number of significant interaction variables was also reduced,
suggesting that taking the log of these variables was doing a better job of accounting for
the interaction effects than the explicit interaction variables. In the log models, only the
parking*daylight hours interaction variable remained significant.
5.3.1. Comparison of Linear versus Log Models
We used a number of criteria to compare the validity of the linear and log models. The
primary criterion was the mathematical “fit” of the models, as expressed in the R2. The
explanatory power, as expressed in R2, of all the models is quite high1. For the linear
models it is 74% (24m) and 80% (10m). In other words, the models are explaining 74-
80% of the variation in sales among the stores studied. This is considerably more than
our previous retail study, which achieved an R2 of 58%.
The R2 of the log models is in a similar range. However, it is not appropriate to compare
the R2 of linear versus log models, since the outcome variables are not defined on the
same scale. An appropriate comparison between models of this type was developed by
statisticians called the “Box-Cox Transformation”.
1
See Appendix 8.3 for an explanation of the R2 expression.
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RETAIL AND DAYLIGHTING STATISTICAL METHODOLOGY
A Box-Cox comparison was run between the linear and the natural log models. Using
this method, it was found that there was virtually no difference in the explanatory power
of the two sets of models. Thus, they were judged equally good at explaining the data.
We then applied secondary criteria to comparing the models including the parsimony of
the models, the consistency between the two time periods, and the reasonableness of
the predicted effects.
• PARSIMONY: The log and linear models were found to be equally parsimonious. In
both cases the 10 month models used 11 variables and one outlier and the 24 month
models used 10 and the same outlier. The log and linear models had degrees of
freedom of 62 and 59 respectively.
• CONSISTENCY: The log and linear models were found to be equally consistent in
their predictions of a daylight effect per store site between the two time periods. This
is illustrated visually in Figure 6, which shows a very consistent pattern of predicted
daylight effects between the two time periods for both types of models.
Daylight Effect on Sales Index per Store, Daylight Effect on Sales Index per Store,
Comparison of 24m & 10m Linear Models Comparison of 24m & 10m Log Models
100% 100%
24m linear log 24m
80% 80%
Percent Change in Sales
Percent Change in Sales
10m linear log 10m
60% 60%
40% 40%
20% 20%
0% 0%
-20% -20%
-40% -40%
-60% -60%
Stores Ranked by Store Number Stores Ranked by Store Number
Figure 5: Consistency of Daylight Effects – Linear vs. Log Models
• MODERATE EFFECTS: The log models predicted slightly more conservative
daylight effects across the range of daylit stores than the linear models. This is
illustrated in Figure 6, which shows the difference in predicted daylight effects
between the log and linear models for the two time periods. Figure 7 reports on the
numerical ranges between the minimum and maximum daylight effects predicted by
the various models. The 24m and 10m log models had predicted ranges that were
77% and 63% respectively of the linear models.
Daylight Effect on Sales Index per Store, Daylight Effect on Sales Index per Store,
Comparison of 24m Linear & Log Models Comparison of 10m Linear & Log Models
100% 100%
24m linear 10m linear
80% 80%
Percent Change in Sales
Percent Change in Sales
log 24m log 10m
60% 60%
40% 40%
20% 20%
0% 0%
-20% -20%
-40% -40%
-60% -60%
Stores Ranked by 24m Log Value Stores Ranked by 10m Log Value
Figure 6: Comparsion of Daylight Effects per Store – Linear and Log Models
27
RETAIL AND DAYLIGHTING STATISTICAL METHODOLOGY
Model Name Min Effect Max Effect Range of Effects
24 m Linear -50% 56% 106%
24 m Log -45% 37% 82%
24m Log range as a percent of 24m Linear range 77%
Model Name Min Effect Max Effect Range of Effects
10 m Linear -46% 88% 134%
10 m Log -35% 49% 84%
10m Log range as a percent of 10m Linear range 63%
Figure 7: Range of Daylight Effects per Store predicted by Linear v Log Models
Based on the secondary criteria of a more moderate range in prediction of effects,
we selected the logged models as the preferred models of the daylight effects of this
retailer, and so the logged models are considered first with greater detail in analysis.
The results of both types of models, however, were remarkably similar and so we will
present the findings about the net daylight effects of both the linear and log
approaches in the next section.
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RETAIL AND DAYLIGHTING ANALYSIS FINDINGS
6. ANALYSIS FINDINGS
In this section we report on the findings primarily from the log models and secondarily,
the linear models. The full equation for each model, and the descriptive statistics for all
the variables considered in the model, are detailed in the Appendix.
As mentioned earlier, some of the variables have been converted into “scalars” in order
to preserve the anonymity of the participant. In all cases, these scalars were created
using simple division or multiplication as appropriate, so that they are statistically
consistent.
To calculate the overall effect of more daylight on corporate sales, we first calculated the
effect of daylight on each individual store, considering all interaction effects specific to
each store. We then summed the effects on all daylit stores, and divided by the sum of
all sales for those stores, to calculate the “net daylight effect,” or the average predicted
effect on sales for adding daylight to any store in the chain.
It is not really appropriate to calculate a standard deviation for these findings, since they
are not based on one yes/no variable, but a multi-dimensional group of variables. In
order to express the range of the potential effect of daylight on an individual store, we
have plotted the range of predicted effects for the two models in Figure 9 and Figure 17,
below. It is important to note that the models do not predict a positive effect for every
individual store. Some stores are predicted to have lower sales associated with
daylighting, based on the effects of the interaction variables.
We performed an additional statistical test to consider the certainty of the net effect
predicted by the combination of interaction variables. We used an F-Test to test the null
hypothesis that the beta coefficients of each of the interaction variables in the group are
simultaneously zero. The alternative hypothesis is that there is at least one variable in
the group where the beta coefficient is not zero. The groups of interaction variables were
found to be significant in all models, and so remain in our final models.
6.1. Log Models
The log models had consistent explanatory variables for both the 10-month and the 24-
month versions, except for one additional interaction variable in the 10 month model.
The magnitude of each variable’s effects and significance are also quite similar. The R2
of the log models are 74.7 and 75.7 respectively. Thus, we are explaining about 75% of
the variation in the sales data between stores, while 25% remains unexplained due to
other factors not considered, or just random variation.
29
RETAIL AND DAYLIGHTING ANALYSIS FINDINGS
Model Name: LN 99, 00 Model Name: LN 01
Variable B Sig. Variable B Sig.
logArea 7.694 0.001 logArea 6.133 0.002
logAge 0.246 0.000 logAge 0.305 0.000
Transport -0.00002 0.000 Transport -0.000014 0.000
Education 0.00001 0.001 Education 0.000004 0.001
Co-mktg 0.091 0.000 Co-mktg 0.072 0.000
Compet 1 -0.056 0.004 Compet 1 -0.047 0.004
Height -0.161 0.023 Height -0.140 0.007
logPark -1.823 0.000 logPark -1.828 0.027
out440 0.651 0.002 out440 0.651 0.002
DayHrs -0.00057 0.003 DayHrs -0.00040 0.003
ParkDH 0.00024 0.002 AgeDH -0.00003 0.092
ParkDH 0.00024 0.015
Model Summary: Model Summary:
RMSE 0.19 RMSE 0.17
R^2 74.7% R^2 75.7%
Figure 8: Results of Log Models
Figure 8 lists the variables this model found to be significant in predicting the sales
index, along with their magnitude and significance. The variable with by far the
strongest positive effect on sales was the size of the sales area. Other variables with
positive effects include the age of the store, the number of sister stores nearby, and a
more educated local population.
In a linear model, a one-unit change in the explanatory variable (X) predicts an
approximate constant, or fixed, unit change (B) in the sales index (Y). So for example, if
the size of the store increases by one square foot, then the sales index will go up by B.
In these log models, a one-unit change in a non-logged explanatory variable predicts an
approximate percentage change in the sales index. And for logged explanatory variables
in the log model, a one-percent increase in the explanatory variable predicts an
approximate percentage change in the sales index. So for example, in these log
models, since square footage was also logged, as the size of the store increases by one
percent then sales index is increased by approximately 6-7 percent.
While the B-coefficient for the Daylight Hours variable appears negative in these models,
the actual net daylight effect turns out to be positive, once the effects of the interaction
variables are taken into account. In order to express the range of the potential effect of
daylight on an individual store, we have plotted the range of predicted effects for the two
Log models in Figure 9 below.
30
RETAIL AND DAYLIGHTING ANALYSIS FINDINGS
Range of Predicted Daylight Effect per Store,
Log Models
50%
40% log 24m
log 10m
Percent Change in Sales
30%
20%
Means
10%
0%
-10%
-20%
-30%
-40%
-50%
Individual Stores Sorted by Magnitude of Effect
Figure 9: Graph of Predicted Range of Daylight Effect per Store, Log Models
The Log Models find that adding daylight to stores (based on the norm of the corporate
design, or about 1090 hours of useful daylight per year, or about ¼ of the total yearly
daytime hours) will be associated with a “net daylight effect” showing an increase in
sales per Figure 10 below:
Model Name Net Effect of Daylight group F-test
Natural log 10 months +5.7% >.01
Natural log 24 months +1.1% >.005
Figure 10: Net Effect of Daylight on Sales, Log Models
Figure 10 shows that the average net effects for the daylight interaction variables as a
group are positive for both models, and that the interaction variables are all significant as
a group (group F-test). This average effect is, however, not large enough in either case
to give certainty that it would not dip down below zero if we considered a different
population of stores in our analysis. A larger population of study sites (for example,
doubling the number of sites from 73 to 150) would have provided greater statistical
power, and would have likely provided greater certainty in the analysis.
Thus, the log models predict a chain-wide average increase in sales associated
with the presence of daylight of 1% to 6%.
6.1.1. Daylight Effect Interaction with Parking
In all of our models, we found that the daylight hours*parking interaction variable was
significant. This means that, for whatever reason, the daylight effect was being modified
by the amount of parking available at each store site. As explained above, we
calculated a net daylight effect for each store site, based on the value of the parking
scalar and daylight hours variable for that site.
Figure 11 plots the predicted daylight effect for each store as a function of its parking
area relative to the norm. It is clear from this graph that as available parking area
increases the predicted daylight effect also rises. The daylight effect starts to go
negative when parking is reduced to 90% or less of the norm for the chain.
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RETAIL AND DAYLIGHTING ANALYSIS FINDINGS
Daylight Effect relative to Parking as a Percent of Norm
60%
50%
Percent Daylight Effect
40%
30%
20%
10%
0%
-10% 0% 50% 100% 150% 200%
-20%
-30%
-40%
Store Parking Area relative to Norm
Figure 11: Daylight Effect relative to Parking Area
In order to understand the theoretical impact of the daylighting effect independent of the
parking interaction variable, we held parking constant. We performed this exercise at
three levels—the norm for the daylit stores, and the norm plus or minus the standard
deviation of parking for the daylit stores—and then predicted the net daylight effect for
the group of daylit stores, as shown in Figure 12. When parking is greater than norm,
the daylight effect jumps up dramatically to +20%. When parking is restricted, the
daylight is seen to have a negative effect. The nature of the linear equations used in the
regression models forces one end of the range to go negative when the other end is
strongly positive, something like a see-saw. Thus, there is less certainty about the high
or low ends of the predicted effect than the norm.
Condition 2001
Parking @ norm +5.6%
Parking @ 1 std. dev. below norm -8.7%
Parking @ 1 std. dev. above norm +19.7%
Figure 12: Daylight Effect Independent of Parking, Log 10 Month Sales, 2001
Figure 12 suggests that the daylighting effect may have its greatest advantage when
there is sufficient parking to take advantage of the additional demand created.
6.1.2. Daylight Effect as a Function of Daylight Hours
Once we understood the interaction of parking with daylighting effect, we looked at the
predicted daylight effect as a function of increasing daylight hours per store, holding the
size of the parking lot constant. This analysis showed that there is clearly a relationship
between more hours of daylight per store and a greater daylight effect on sales.
This is a clear dose/response relationship, which says that as the number of daylit
hours increases, the relative effect on sales also increases.
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RETAIL AND DAYLIGHTING ANALYSIS FINDINGS
The 2001 model suggests that, when parking is held constant at the mean, for every
increase in 100 daylight hours per year per store, the daylight effect increases by 1%,
ranging from a low of -2% to a high of +14%. When parking is held constant at a high
level (the mean plus one standard deviation), then for every increase in 100 daylight
hours per year per store, the daylight effect increases by 2.4%, ranging from +2% to
+37%. These predictions are illustrated in Figure 13.
In the 1999-2000 model, when parking is held constant at the mean, an increase in 100
hours of daylight increases the daylight effect by 0.1%, ranging from +0.4% to +2.0%.
When parking is held constant at a high level (the mean plus one standard deviation),
then for every increase in 100 daylight hours per year per store, the daylight effect
increases by 2%, ranging from +5% to +27%. These predictions are illustrated in Figure
14. (Note that in Figure 14 the scale for one graph, “1999-2000 parking at mean model,”
has a different vertical scale, 1/10 the size of the others, in order to show detail in the
smaller effects seen in this example.)
The results for these plots suggest a “bounds” for a daylight effect. When parking is held
at norm, the daylight effect varies from a low prediction of –2% to a high of +14%
increase in sales per store. In these equations, the amount of parking becomes a
limiting factor. When we allow parking to increase up to a higher level, one standard
deviation above the norm, the prediction of the daylight effect per store ranges from +2%
to +37%. In all cases, as the number of useful daylight hours per year increases, the
relative daylight effect on sales also increases.
2001 Ln Daylight Effect, Parking Held Constant (mean) 2001 Ln Daylight Effect, Parking Held Constant (high)
50.0% 50.0%
y = 0.0002x - 0.0426
40.0% 40.0%
R2 = 0.566
30.0% 30.0%
Daylight Effect
Daylight Effect
20.0% 20.0%
y = 0.0001x - 0.0429
R2 = 0.2162
10.0% 10.0%
0.0% 0.0%
0 500 1000 1500 2000 0 500 1000 1500 2000
-10.0% -10.0%
-20.0% -20.0%
Daylight Hours Daylight Hours
Figure 13: Daylight Effect as a Function of Daylight Hours, Log 10 Month Sales, 2001
9900 Ln Daylight Effect, Parking Held Constant (mean) 9900 Ln Daylight Effect, Parking Held Constant (high)
5.0% 50.0%
4.0% 40.0%
y = 1E-05x + 0.0012 y = 0.0001x + 0.0071
R2 = 0.7946 R2 = 0.8134
3.0% 30.0%
Daylight Effect
Daylight Effect
2.0% 20.0%
1.0% 10.0%
0.0% 0.0%
0 500 1000 1500 2000 0 500 1000 1500 2000
-1.0% -10.0%
-2.0% -20.0%
Daylight Hours Daylight Hours
Figure 14: Daylight Effect as a Function of Daylight Hours, Log 24 Month Sales, 1999-
2000
33
RETAIL AND DAYLIGHTING ANALYSIS FINDINGS
6.2. Linear Models
For completeness we present the findings of the linear models for comparison to the log
models described above.
The linear models had the same set of explanatory variables as the log models,
consistent across both the 10 month and the 24 month versions, and the same
additional interaction variable in the 2001 model.
The R2 of the linear models are 76.5 and 75.3 respectively. Thus, again we are
explaining about 75% of the variation in the sales data between stores, while 25%
remains unexplained due to other factors, or is just random variation. As explained
earlier in Section 5.3.1, the R2 of log and linear models cannot be compared directly,
since their outcome variables are on different scales. A comparison was done via a
Box-Cox transformation, and the explanatory power of the two equations was found to
be essentially identical.
Model Name: Linear 01 Model Name: Linear 99-00
Variable B Sig. Variable B Sig.
Area 1052 0.002 Area 1305 0.000
Age 147 0.000 Age 111 0.000
Transport -0.038 0.010 Transport -0.064 0.000
Education 0.009 0.007 Education 0.013 0.000
Co-mktg 181 0.001 Co-mktg 217 0.000
Compet 1 -122 0.006 Compet 1 -130 0.006
Height -416 0.007 Height -389 0.019
Parking -579 0.000 Parking -517 0.000
Out44 2183 0.000 Out44 1982 0.000
DayHrs50 -1.41 0.002 DayHrs -1.57 0.001
AgeDH -0.08 0.089 ParkDH 0.64 0.001
ParkDH 0.73 0.000
Model Summary: Model Summary:
RMSE 440 RMSE 475
R^2 76.5% R^2 75.3%
Figure 15: Results of Linear Sales Model
Here the B-coefficients for the Daylight Hours variable are again negative, but once the
effect of the interaction variables are taken into account, the net daylight effect becomes
positive in the 10 month model. In the 24 month model it is found to be effectively zero,
as described in Figure 16 below. In order to express the range of the potential effect of
daylight on an individual store, we have plotted the range of predicted effects for the two
linear models in Figure 17 below.
34
RETAIL AND DAYLIGHTING ANALYSIS FINDINGS
The linear models find that adding daylight to stores (based on the norm of the corporate
design, or about 1090 hours of useful daylight per year) will be associated with the
following increase in sales:
Model Name Net Effect of Daylight group F-test
Linear 10 months +5.2% >.0001
Linear 24 months -0.3% >.005
Figure 16: Net Effects of Daylight on Sales, Linear Models
Figure 16 shows that the predicted net effects are positive for the ten month model, but
slightly negative (or essentially zero) for the twenty-four month model. The interaction
variables are all significant as a group (group F -test) in both models, and so were
retained as a group. The linear and log models thus have essentially the same
prediction: that during the 10 month period during 2001 daylit stores increased their
sales relative to non daylit stores by 5-6%. During the 24 month period during 1999-
2000 the daylit stores were found to be selling at very similar levels to the non-daylit
stores (i.e., 0% to 1%).
In order to express the range of the potential effect of daylight on an individual store, we
have plotted the range of predicted effects for the two linear models in Figure 17 below.
Range of Predicted Daylight Effect per Store,
Linear Models
100%
90%
24m linear
80%
Percent Change in Sales
70% 10m linear
60%
50%
40%
30%
20%
Means
10%
0%
-10%
-20%
-30%
-40%
-50%
Individual Stores Sorted by Magnitude of Effect
Figure 17: Graph of Predicted Range of Daylight Effect per Store, Linear Models
Given the similarity of results, we did not attempt to describe the effect of daylight
independent of parking, as we did above with the log models.
Again, per the discussion in the log models, this average effect is not large enough to
give certainty that it would not dip down below zero (in the case of the ten month model)
or rise above zero (in the case of the twenty-four month model) if we considered a
different population of stores in our analysis. A larger population of study sites (say
doubling the number of sites from 73 to 150) would have been helpful to provide greater
certainty in the models’ predictions.
35
RETAIL AND DAYLIGHTING ANALYSIS FINDINGS
6.3. Discussion of Findings
These statistical models are substantially more complex than considered in the previous
study, and give less dramatic results. However, we actually have higher confidence in
this analysis, given the amount of attention given to verifying details of the data, and
testing alternative hypotheses. The smaller magnitude of the predicted daylight effect is
actually closer to what one would intuitively expect. Indeed, the very size of the
prediction of the previous study (that daylit stores were selling 40% more than non-daylit
stores) made it subject to criticism and disbelief.
This analysis has also provided us with a rich field of information that has allowed us to
extend the results into more detailed consideration of the implications of daylighting and
illumination on sales.
6.3.1. Variable R2 and Order of Entry
The order of entry of variables into the model and the amount of variance explained by
each variable (partial R2) can be an important indicator of the relative importance of a
variable in predicting an outcome. We show these statistics for our four models in the
Appendix in Figure 24, Figure 25, Figure 29, and Figure 30.
In both the log and the linear models, the age of the store is consistently the most
important predictor of sales for this chain, explaining from 28% to 38% of the variance in
the sales data. All of the rest of the variables in the model are considerably less robust,
predicting less than 8% of the variance, and often less than 1%. The size of the store
and the amount of parking tend to be the two next most powerful variables, at partial
R2 = 5-8% and 4-8% respectively. It is interesting to note that the daylight hour variable
tends to be the next most powerful predictor of sales (4%-5%), consistently at least as
strong as, if not stronger than, the competition variables (3%-6%) and than the
demographic variables from census data (1%-6%). This means that information about
the amount of daylight hours in a store is doing at least as good of a job, if not better, of
explaining variation in store sales than information about the number of nearby
competitors or the population demographics of the neighborhood.
Thus, these models strongly suggest that the amount of daylight in a store is
equally useful in explaining sales potential as the more traditional
characteristics—parking, competition and demographics—to which classic real
estate analysis pays a great deal of attention.
6.3.2. Comparison with Previous Study
The retailer in this study had a less aggressive daylighting design strategy and also
substantially more variation in both the range of daylight conditions and the range of
store designs than the retailer in the first study. On the one hand, the greater range of
conditions, combined with a smaller number of study sites, would suggest that we would
not be as successful in predicting the influences on sales as with the previous
participant, who maintained greater uniformity in their operations. On the other hand, the
greater range of information and the presumed greater accuracy of the information,
given the attention to site verification, would suggest that we should have greater
success in predicting influences on sales. The R2 of the models suggest that the second
trend was stronger, as the models of this study explain about 75% of the variation in the
sales data compared to 58% in the previous study.
36
RETAIL AND DAYLIGHTING ANALYSIS FINDINGS
The attempt to replicate the format of the previous model was most likely unsuccessful
because of two characteristics of the stores in this study. First of all, the average
daylight effect was observed to be much smaller, i.e. closer to zero, and therefore less
likely to be found significant in a simple yes/no model. The greater sensitivity of a scalar
description of daylight, describing the number of hours per year of daylight above a
certain threshold helped to provide better resolution of the relationship between daylight
and sales.
Secondly, for this particular chain there seems to be an important interaction between
the amount of parking and any daylight effect. We do not know why this is so – we can
only hypothesize based on common sense why such an interaction might occur. There
is also always the possibility that this statistical interaction is simply a random
occurrence of how the types of stores are distributed in the data. In the previous model
we had no information about the amount of parking available at the various stores.
Parking was not considered as an explanatory variable in the original model. Thus, we
have no way to compare this parking-daylight effect with information from the previous
participant.
The use of the daylight hours variable in this set of models has allowed us to detect a
more subtle effect of daylight on sales, and also to describe a dose/response
relationship between more daylight and more sales. This dose/response relationship is
inherently more useful information to designers. It basically says that “more is better.” It
is not simply the presence of daylight at some threshold level, but progressively
increased exposure that is most useful. It is unclear, however, from this analysis
whether the increase has to do with more hours of useful daylight or higher levels of
daylight illumination, since the two go hand in hand and we cannot distinguish between
the two characteristics in the stores we studied.
6.3.3. Comparison of 10 Month and 24 Month Time Periods
It is interesting to consider why there was a significant daylight effect observed for the 10
month period and not for the 24 month period. There are at least two possible
explanations for this finding, which we will call the “contrast” hypothesis, where daylit
stores gain in comparison to sister stores, and the “competitive” hypothesis, where daylit
stores are more likely to gain competitors’ business.
On the one hand, we separated the time periods in the analysis specifically because
they had different lighting operation conditions. In the 24 month period (1999-2000)
electric lights in the non-daylit stores were on at full power at all times, while lights in the
daylit stores were controlled to respond to daylight. During the 10 month period (2001)
the electric lights in all stores were at reduced levels, both day and night. As a result,
there was a greater contrast in ambient light conditions between daylit and non-daylit
stores during the 10 month period.
The contrast hypothesis suggests that the greater daylight effect observed during the 10
month period was partly caused by the greater contrast in illumination levels between
daylit and non-daylit stores. If daylit stores are observed to be selling more than non-
daylit stores during the power reduction, it might be tempting to argue the alternative:
that the reduction in lighting power during the 10 month period “hurt” sales for the non-
daylit stores. However, this does not seem to be the case since all stores in the chain
increased their sales during the 10 month period. Something in the general economy (or
perhaps, store management) seems to have increased sales for all participant stores.
37
RETAIL AND DAYLIGHTING ANALYSIS FINDINGS
The contrast hypothesis focuses on the differences between sister stores within the
same chain. But, from the corporation’s perspective, each store site is more importantly
competing with the competitor’s stores. Differences between this chain and other chains
are more likely to be important determinants of corporate success than differences within
the chain. During the California power crisis of 2001 almost all retailers in the state
agreed to operate their stores at reduced lighting power in order to conserve energy and
reduce peak loads on the state electric system. As a result, not only the study
participant but most of their competitors were also operating at reduced electric lighting
levels. Under these conditions, the daylit participant stores were even more successful
than the rest of the chain.
The competitive hypothesis suggests that under favorable economic conditions, the
daylit stores in the study were even more attractive relative to the competitors’ options
than the non-daylit stores. Could it be that when shoppers are motivated to buy more
products from the participant, they are even more motivated by the daylit stores? During
the California power crisis of 2001 almost all retailers in the state agreed to operate their
stores at reduced lighting power in order to conserve energy and reduce peak loads on
the state electric system. As a result, not only the study participant but most of their
competitors were also operating at reduced electric lighting levels. Under these
conditions, the daylit participant stores were even more successful than the rest of the
chain.
Because both events—the overall favorable economic conditions and the reduced
lighting levels—seem to have happened simultaneously, we cannot distinguish between
possible effects. Research into economic conditions that may have supported one these
hypotheses was outside the scope of this report. Of course, other possible differences
may exist between the two time periods that we did not observe or consider.
6.3.4. Other Findings
The details of the four models are shown in the tables in the Appendix. In simple
English, the models tell us that the following variables, out of all of those we considered,
are the best predictors of how much a given store operated by this retailer will sell:
• Bigger stores sell more
• Stores that are open longer hours sell more (or stores that sell more, are chosen to
stay open longer)
• The older the store, the more it sells
• When the local population spends more time commuting to work, the lower the sales
• When the local population is more educated, the higher the sales
• The more sister stores within a certain radius, the higher the sales for all
• The more competitors within a certain radius, the lower the sales
• The higher the store front, the lower the sales
• The more parking spaces, the lower the sales
• The more hours of useful daylight per store per year, when combined with ample
parking, the higher the sales
38
RETAIL AND DAYLIGHTING ANALYSIS FINDINGS
In understanding these model predictions, it is just as important to look at which
explanatory variables were not found to predict sales. Somewhat surprisingly, only 2 of
the 10 demographic variables tested were found to reliably predict sales. We conducted
a test, described in Section 5.2.4, to see why this was the case. It was fairly clear that
other variables that described the competitive environment—number of sister stores and
number competitors within a certain radius—were already accounting for most of the
demographic influence on sales.
It is also interesting, that of all the information we gathered on-site about the physical
conditions of the stores, only daylighting was found to be reliably significant. Specifically,
luminaire type, air movement, odors and noise were not found to predict sales. It could
be that there was not enough variation between stores on these characteristics for
meaningful analysis. Indeed, this chain makes great efforts to promote uniformity among
stores. It could also be true that the conditions of these variables that our surveyors
observed were not consistent over time or that we did not define the measurement
protocol sufficiently accurately to define the response. Other variables associated with
daylight, such as ceiling height, ceiling type, and vertical illumination levels, were also
found to be positive and significant in earlier models, but daylight hours was found to be
a better predictor then each of them, so they were dropped from the final models.
Storefront Height and Parking
In reviewing these results, the two results most controversial and counter-intuitive for the
corporate managers were that higher storefronts and more parking were associated with
reduced sales. They believe that higher storefronts and more parking should increase
sales. Thus, they proposed that our findings for these variables might be a function of
collinear effects with age or location. However, when we checked for interaction between
these variables and age or demographics, the results still held. The storefront and
parking variables are quite robust, and appeared highly significant in every model we
have tried.
Alternatively, we propose that these findings might be a function of corporate decisions
made relative to perceived competition in the area. When a site is perceived to have a
great deal of competition from other chains that have high store fronts, then a new store
site for this chain will be more likely to be designed with a high store front. Likewise, if
competitors have very large parking lots, then a new store site with larger parking area
would likely be preferred over a smaller site. Since these types of decisions would be
made in a more competitive environment, it might be that they are associated with
reduced sales due to the character of the local competition, rather than to a direct
cause/effect relationship associated with the store front or the parking area.
Alternatively, it might be hypothesized that store sites with parking areas larger than
norm were likely to be located in less successful shopping centers, for two possible
reasons. One, lower pressure on land prices due to less retail activity might encourage
the establishment of larger parking lots. Or two, highly successful shopping centers
might be more likely to add additional stores to the complex, thereby reducing the
availability of parking for the group of tenants as whole. Indeed, sites for future additional
stores are often reserved in new shopping centers, but are maintained as overflow
parking until the demand for new stores arises. Our methodology would not have
distinguished the difference between normal (required) parking and parking areas
designated for future store sites. Thus, less successful shopping centers are unlikely to
fill in their extra store sites and therefore would be counted as having larger overall
parking areas.
39
RETAIL AND DAYLIGHTING ANALYSIS FINDINGS
Interaction Variables
The interaction variables are also of interest. It was clear from the start of the study that
this retailer had a much greater variety of store conditions than our previous study
participant, and that a number of store characteristics were strongly associated with the
presence of daylight. The interaction variables account for these interactions and
moderate the effect of daylight relative to the presence of these other influences on
sales that are correlated to daylight. The interaction variables certainly make for a more
complex model, but also help us describe a more nuanced reality.
The interaction of parking with daylight hours is perhaps the least expected. As a simple
variable, more parking, relative to all other variables, is seen to have a negative effect on
sales. However, when we add in the interaction variable parking*daylight, more parking
increases the positive effect of more daylight. This is true in both the log and linear
models. This suggests that the negative effect of more parking is slightly overstated in
the simple variable, and is moderated in the daylit stores.
6.4. Additional Analysis
We were able to perform some additional analysis to clarify secondary issues, using the
same data sets. The following sections report on models which looked at potential
seasonal variations in a daylight effect and a potential effect of daylight on the number of
transactions per store.
6.4.1. Seasonal Effects
As part of our initial analysis, we also looked at the seasonal difference between daylit
and non-daylit stores. This was first done simply by comparing the difference in monthly
sales averages between the two types of stores. No other control factors were included.
A plot of the difference in sales between the two groups of stores showed no obvious
seasonal pattern. Indeed, it seemed to be rather random. There was no evidence of
increasing or decreasing sales due to predictable changes in the climate, solar intensity,
or outdoor temperature.
It was still possible, however, that a more sophisticated, multivariate seasonal analysis
might turn up seasonal differences in sales performance between the two types of
stores. To test this theory, we created a seasonal model using all of the variables
considered in the sales models. Instead of yearly average sales indexes as outcome
variables, we used the monthly average sales indexes for two extreme seasonal
conditions—July and January—for all three years. July represents some of the longest,
sunniest days of the year, and January represents some of the shortest, cloudiest days
of the year. Using these two months also allowed us to avoid any anomalies due to the
holiday period in December and include three years of observations for each season.
This model included two additional types of explanatory variables; an indicator variable
for each year and an indicator variable for month. These monthly models basically
made the same predictions as the larger yearly models, with no hint of a variation in
daylight effect between the two months. We reasoned that with no suggestion of
seasonal variation between these two extremes, it was not worthwhile pursuing further
efforts to find a seasonal effect.
40
RETAIL AND DAYLIGHTING ANALYSIS FINDINGS
Given our null finding of a seasonal difference in a daylight effect, we conclude
that any “daylight effect” is more likely to be a result of long-term impact on
overall customer loyalty, affecting sales through out the year, rather than a short-
term boost in sales due to higher illumination levels or longer daylight hours.
6.4.2. Number of Transactions
Since we were given information about the number of transactions per store in addition
to the value of sales, we decided to test the hypothesis that daylight increased sales by
increasing the number of transactions rather than the value of sales per transaction. A
transaction for this purpose is counted as one store visit per customer which resulted in
sale of any number of items. Thus, an increase in the number of transactions at a store
site could result from either an increase in the number of customers or an increase in the
number of visits per customer, or both. As with the sales information, all of the
transaction data was transformed into a dimensionless index for the analysis to preserve
confidential information. Since the sales index and transaction index use different
transformations, their values in the models cannot be compared.
We created linear regression models using all the explanatory variables considered in
the sales index models. The findings of the transaction models are shown in Figure 31
and Figure 32 in the Appendix. The R2 for the ten and twenty-four month models were
0.77 and 0.75 respectively. The models used the linear format, and both include
interaction variables, similar to the sales models, thus the average effect of daylight on
number of transactions for the chain as a whole must be predicted by averaging the
combined interaction effects for each store.
Model Name Net Effect of Daylight group F-test
Linear 10 months +2.1% >.005
Linear 24 months +1.2% >.005
Figure 18: Net Effect of Daylight on Number of Transactions per Store
Figure 18 shows that the chain-wide average net effect of daylight is positive for
both models, ranging from a 1% to 2% increase in number of transactions.
The interaction variables are all significant as a group (group F-test) in both models, and
so were retained as a group. The magnitude of the predicted increase in transactions is
modest, and somewhat less than the prediction for the increase in sales for the
comparable linear 10-month model. Thus it is likely that the “daylight effect” is working
both to increase the amount of traffic through the stores, as evidenced by the increase in
the number of transactions, and also to increase the value of each set of purchases, as
evidenced by the relatively greater sales effect than transaction effect.
Somewhat surprisingly, the models maintained an almost identical format as the sales
models, with all of the same explanatory variables being retained, with the exception of
the demographic variables. In the sales transaction models, the demographic variable
housing was significant for both the ten month and twelve month models, and the
variables for population growth and transportation were significant in only the ten month
model. The inconsistency of the demographic variables once again argues that they are
slightly less reliable predictors of sales (or in this case, number of transactions) than the
other variables which are consistent across all models. The consistency of daylight in
41
RETAIL AND DAYLIGHTING ANALYSIS FINDINGS
predicting a positive effect based on a different outcome variable once again increases
our confidence that it is likely to be a true effect.
We did go through the exercise of isolating the effect of daylight independent of its
interaction with size of parking, as we did above with the log sales models, and found a
similar pattern; when parking is at or above norm, an increase in the number of useful
daylight hours per stores is also associated with an increase in the percentage effect on
transactions.
Again, per the discussion in the log sales models, this average effect is not large enough
to give certainty that it would not dip down below zero if we considered a different
population of stores in our analysis. A larger population of study sites (say doubling the
number of sites from 73 to 150) may have provided greater certainty in the models’
predictions.
42
RETAIL AND DAYLIGHTING OTHER STUDY FINDINGS
7. OTHER STUDY FINDINGS
In addition to the regression analysis of sales and number of transactions per store,
which form the core of this research, we also looked at the potential energy impacts of
the daylighting system and assessed employee and store manager satisfaction with the
daylighting design. We assessed the energy impacts to quantify the rather predictable
dollar value of energy savings due to skylights combined with automatic photocontrols,
which will reliably occur in addition to any sales impact. We looked at employee and
manager satisfaction with the daylighting as a way to try to get insight into the causal
mechanisms of any daylighting effect, and also to identify any problems that might be
associated with the daylighting systems in the stores.
The analysis of energy impacts were based on both interviews with the corporate
management and our own estimates of energy savings based on store characteristics
and operation schedules. The energy estimates are not based on monitoring. The
assessment of employee and manager satisfaction with the daylighting system was
based on interviews with the managers and a formal survey distributed to employees.
The following sections present our findings in these two areas, and discuss their
relevance to the overall study.
7.1. Energy Impacts
Energy savings were the primary motivation for both the original installation of skylights
with photocontrols, and the one-half lighting power reduction during the 10-month study
period. Both of these programs resulted in substantial dollar savings for the retailer. The
retailer is very satisfied with the resulting energy savings and considers these savings to
be an important reduction in operating costs affecting the bottom-line profitability for the
chain.
7.1.1. Store and Corporate Energy Impacts
The energy savings achieved by this chain are a result of the use of automatic
photocontrols that reduce lighting energy use when there is sufficient daylight available
in the stores. Longer hours of useful daylight (above threshold) per day result in greater
energy savings.
We did not monitor operation of the photocontrols or the overall energy performance of
whole building systems relative to the skylight impacts. We did however, calculate
lighting and whole building energy savings using SkyCalc and DOE-2 computer
simulation models of the daylit stores, and compared these findings to average energy
expenditures for the retailer during the two time periods.
The lighting energy savings from the skylights and photocontrol operation tend to run
from about 20% to 30% compared to electric lights on at full power, while the whole
building (lighting and HVAC) energy dollar savings range from about 15% to 25%.
These numbers all vary by climate, daylighting system and store design, and the
photocontrol settings and operation. The stores are not necessarily using optimized
designs, so potential savings due to the daylight could be higher with different design
choices.
43
RETAIL AND DAYLIGHTING OTHER STUDY FINDINGS
We calculated the energy savings from the current design and operation and then
gradually increased the optimum performance of the skylight and photocontrol system
heading towards a theoretical maximum performance. We found that the current system
(good) is saving about $.24/sf for an average store in the chain, while an improved
system (better) using current best-practices could save about $.54/sf, and an optimum
system (best) using state-of-the-art performance could save about $.66/sf at current
energy prices. Thus, the current daylight design is saving about one-third of the
maximum amount of energy that could potentially be saved from daylighting.
7.1.2. Statewide Energy Impacts
Applying skylights with automatic photocontrols to new and remodeled retail buildings in
California has a potential to provide considerable energy and power savings in the state.
California adds about 84.8 million sf of new commercial space each year, of which 4% is
groceries and 16% is other retail1. This adds up to 17 million sf of new retail construction
per year. In California the vast majority of this retail space is single story construction.
We know from other sources that 46% of retail space nation wide uses hung ceilings,
while 54% uses exposed ceilings2. If we assume that of spaces with hung ceiling 50% of
the total area could be realistically skylit, and that of the spaces with exposed ceiling the
rate is higher, at 75% of the area, then we estimate that there is 10.8 million sf per year
that could potentially include skylighting.
If we apply the energy and dollar savings achieved by the average store described
above across the whole state, then the value of this savings would be $2.5 million dollars
per year, or 13.2 megawatt-hours per year3. After the end of ten years of construction,
the value would potentially increase ten fold, to $25 million per year, and 132 megawatt-
hours4.
However, as discussed above, the average store does not have an optimum skylight and
photocontrol system. If we applied the “better” design, using current best-practices
components, this value could be increased to $5.8 million per year or 41.6 megawatt-
hours. At the “best” level, with state-of-the-art components capturing the maximum
technical potential, these numbers could increase to $7.1 million per year or 58.4
megawatt-hours per year. Again these values should be multiplied by a factor of 10 to
get the value after ten years of construction.
The above calculations assume skylights are added only to new buildings. If a retrofit
market for skylights and automatic photocontrols developed, these values would
potentially increase by about another 50%.
1
Brooks, M. 2002 “California Electricity Outlook: Commercial Building Systems” Presentation at PIER
Buildings Program HVAC Diagnostics Meeting, Oakland, CA, April 16.
2
Armstrong Industries, 2002, private communication.
3
These values are based on SkyCalc® runs, which account for lighting, heating and cooling savings, and
combine the net annual value of electricity and gas impacts into a blended kWh value.
4
It is not possible to translate megawatthours into peak megawatt impacts, since the dynamics of climate
and electric peaks greatly complicate the equation. A separate study should be done to understand the
potential peak impacts of skylighting systems on state power demand.
44
RETAIL AND DAYLIGHTING OTHER STUDY FINDINGS
7.1.3. Energy Impacts Relative to Daylight Effect on Sales
With each of these steps of daylight system performance improvement, the hours of
daylight above threshold also increases. Thus, according to our model of sales
performance, the daylight sales effect would also increase. To compare energy savings
to sales impacts, we also calculated the progressive increase in sales impacts due to an
improved daylighting system, making conservative assumptions about the value of sales
per square foot, and assuming a store with average conditions for both daylight and
parking. We found that while the sales effect increased with an improved daylighting
design since there would be more hours of useful daylight per year, the energy savings
increased at an even faster rate. For the 24 month period, the ratio of the value of the
daylight sales effect to the energy savings was 45 times at the “good” (existing) level, 22
times at the “better” level and 19 times at the “best” level. For the 10 month period the
sales numbers increase dramatically, since a higher value was found for the daylight
effect. Under the 10 month conditions the ratio of daylight effect on sales to daylight
energy savings was 234 times at the good level, 124 times at the better level and 107
times at the best level.
Thus, for a daylighting design of the current (good) performance,
the value of the daylight effect was estimated at 45 times greater than the value of
the energy savings, using the conservative estimate of a 1.1% sales effect from
the 24-month period.
With a fully optimized energy design, with three times the energy savings, this ratio is
still maintained at 19 times. Should the much higher 5.7% sales effect from the 10-month
period apply, the predicted value of additional sales is worth more than 100 times any
energy savings.
7.2. Employee Assessment of Lighting Quality
Employees in all surveyed stores were asked to fill out a brief survey on their personal
assessment of the lighting quality in the store. We used the same lighting quality
assessment instrument that we have used in previous surveys for Southern California
Edison, based on an instrument originally developed by Dr. Peter Boyce at the Lighting
Research Center in Troy, New York. Our survey asked employees to rate their opinion
of the store’s current lighting conditions, on a scale of 1 to 7, where 1 is “I strongly
disagree” and 7 is “I strongly agree,” or 1 is “much worse than norm” and 7 is “much
better than norm, depending on the nature of the question. We received 1128
responses from an average of 18 employees in 62 out of 73 of the stores studied.
We then compared the responses of employees in daylit versus non-daylit stores. For
all questions, employees in the daylit stores rated all aspects of lighting quality slightly
better than those in the non-daylit stores. The responses to the various questions gave
daylit stores higher ratings ranging from 1% to 9%, with an average of 5% fewer
reported problems. The overall assessment was that the daylit stores were 8% better lit
than non-daylit stores within the chain, and also 8% better lit than all comparable stores.
Those answers with 5% or greater percentage difference have more than 90% certainty
(p<0.10).
The comparison between the two types of stores is summarized in Figure 19 below.
45
RETAIL AND DAYLIGHTING OTHER STUDY FINDINGS
1. Overall, the lighting quality in this store is comfortable.
*Daylit stores rated 5% better than non-daylit stores
2. The lighting helps make the merchandise look appealing.
Daylit stores rated 1% better than non-daylit stores
3. The store is uncomfortably bright.
Daylit stores rated 2% better than non-daylit stores
4. The store is uncomfortably dim.
*Daylit stores rated 8% better than non-daylit stores
5. The light fixtures themselves are too bright.
Daylit stores rated 2% better than non-daylit stores
6. There is too much light in some areas and not enough in others.
*Daylit stores rated 9% better than non-daylit stores
7. The lighting makes it difficult to examine detail closely.
*Daylit stores rated 6% better than non-daylit stores
8. Reflections on the merchandise are sometimes a problem.
*Daylit stores rated 6% better than non-daylit stores
9. Skin tones look unnatural under this lighting.
Daylit stores rated 3% better than non-daylit stores
10. It is difficult to distinguish shades of color under this lighting.
*Daylit stores rated 6% better than non-daylit stores
11. The lights sometimes flicker or hum annoyingly.
Daylit stores rated 4% better than non-daylit stores
12. How does the lighting in this store compare to lighting in similar stores?
*Daylit stores rated 8% better than non-daylit stores
* indicates p<.10, statistical certainty greater than 90%
Figure 19: Employee Assessment of Lighting Quality
The biggest specific differences reported between the two store types were that:
Daylit stores overall were less uncomfortably dim (8%)
Daylit stores had fewer problems with uniformity (9%)
When these surveys were taken, the daylit stores tended to have higher illumination
levels than the non-daylit stores, and indeed employees reported fewer problems with
the stores being “uncomfortably dim”.
It is especially interesting that the employees reported that the daylit stores had
better uniformity by 9% (“There is too much light in some areas and not enough in
others”), even though we measured substantially bigger variation in illumination
levels in the daylit stores than in the non-daylit stores (the standard deviation of
both horizontal and vertical illumination was four times greater in the daylit
stores).
46
RETAIL AND DAYLIGHTING OTHER STUDY FINDINGS
7.3. Manager Assessment of Lighting Quality
During the 24-month period store managers assessed the lighting quality in both the
daylit and the non-daylit stores to be equally high (86% and 83% respectively). During
the 10-month period, many more (2.5x) managers thought the lighting quality in the
daylit stores was good or superior than did managers in non-daylit stores.
Managers were also asked if they thought that the lighting quality in their store had a
positive effect on sales. Managers of both daylit and non-daylit stores had generally the
same opinions. The big difference was between the 10-month period at one-half power
and the 24-month period at full power. During the 24-month period, three times as many
managers in the non-daylit stores and twice as many managers in the daylit stores
thought that the lighting quality had a positive effect on sales, compared to their answers
relative to the 10-month period.
Interestingly, the managers’ intuitive attitudes contradict our findings. We found both
types of stores had higher sales during the 10-month period, and that the daylit stores
always had higher sales than the non-daylit stores. The difference in sales between
daylit and non-daylit stores was also greater during the 10-month period.
Mangers were asked if they had any comments to volunteer about the lighting at one-
half or full power and about their experiences with the skylights and photocontrols.
Fifteen out of 73 managers either commented negatively about the reduced light levels
or responded that full light levels were better. Three out of the 73 managers volunteered
that they found the one-half light levels superior. Two of those three were in skylit stores.
Only two volunteered that they thought the energy savings from running the lights at
one-half power was important.
Relative to the skylights, six out of the twenty-four managers in skylit stores made
strongly positive comments about the skylights, and only one made a negative comment.
When asked about specific problems that had ever occurred in the past (indefinite time
period), four reported a skylight had leaked once, and one reported that one skylight had
been broken for one day. None reported any problems with criminal break-ins or
accidental falls, problems with skylights that are sometimes cited as concerns by
building owners who are considering the use of skylights.
It should be noted that store managers tend to be focused on their own stores, so they
may not be able to make meaningful comparisons to other stores with different
conditions.
Thus, we found that store managers tended to have either a neutral or a positive
attitude towards the daylighting system. They did not report problems of any
greater magnitude than might be expected of any other building function.
47
RETAIL AND DAYLIGHTING OTHER STUDY FINDINGS
48
RETAIL AND DAYLIGHTING CONCLUSIONS AND DISCUSSION
8. CONCLUSIONS AND DISCUSSION
This study represents evidence that a major retailer is experiencing higher sales in daylit
stores compared to similar non-daylit stores. In addition to replicating the findings of the
previous study, this study added other important dimensions of information:
• Average effect of daylighting on sales for all daylit stores in this chain was variously
calculated at 0% to 6%, depending on the type of model and time period considered.
• A dose/response relationship was found, whereby more hours of useful daylight in a
store are associated with a greater daylight effect on sales.
• A bound of an empirical daylight effect for this chain was detailed, with a maximum
effect found in the most favorable stores of about a 40% increase in sales. This
upper bound is consistent with our previous finding.
• Daylight was found to have as much explanatory power in predicting sales (as
indicated by the partial R2) as other more traditional measures of retail potential,
such as parking area, number of local competitors, and neighborhood
demographics.
• Along with an increase in average monthly sales, the daylit stores were also found
to have 1-2% increase in the number of transactions per month.
• No seasonal patterns to this daylight effect were observed.
• The value of the energy savings from the daylighting is far overshadowed by the
value of the predicted increase in sales due to daylighting: by the most conservative
estimate of at least 19 times, and more likely, under current conditions, by 45-100
times.
• During the California power crises, when almost all retailers in the state were
operating their stores at half lighting power, the stores in this chain with daylight
were found to benefit dramatically, with an average 5.5% increase in sales relative
to the other stores in the chain (which also increased their sales compared to the
previous period).
• Employees of the daylit stores reported slightly higher satisfaction with the lighting
quality conditions overall than those in the non-daylit stores. Most strikingly, they
perceived the daylit stores to have more uniform lighting than the non-daylit stores,
even though direct measurements showed the daylight stores to have much greater
variation in both horizontal and vertical illuminance levels.
• Store managers did not report any increase in maintenance attributable to the
skylights.
• The chain studied was found to be saving about $0.24/sf per year (2003 energy
prices) due to use of photocontrols, which could potentially increase up to $0.66/sf
per year with an optimized daylighting system.
We were allowed extraordinary access to the store sites and employees, and have had
the opportunity to review the methodology, findings and conclusions with the retailer
management. Thus, we have high confidence in the validity of source data, and the
reasonableness of these findings.
49
RETAIL AND DAYLIGHTING CONCLUSIONS AND DISCUSSION
8.1. Comparison with Previous Retail Study
In this study we have attempted to make a much more detailed study of the relationship
between daylight and retail sales than the previous study. Two major differences existed
between this study and the previous study. First, in the current study, we were able to
account for many more explanatory variables, including radius-based census data,
marketing conditions and other physical conditions of the stores, with on-site verification
of physical conditions at all sites. The second major difference is that in this study we
described the daylight variable as a scalar rather than a simple yes/no variable.
The retailer participant for this study had a greater range of daylighting and climatic
conditions than the previous study participant, whereas the previous study had
implemented a highly standardized store and daylight design. This range of conditions
allowed us to create a scalar variable for the presence of daylight, based on the number
of hours of daylight above a certain illumination threshold per year. Upon analysis, we
also discovered that there were a number of collinear relationships between daylighting
hours and other conditions at the stores. In the previous study we did not have the
resources to study all the interactions of explanatory variables. In the previous study we
had no information about parking, which proved to be highly significant in this study and,
of particular importance, to interact with the predicted daylight effect.
We had a smaller study population in this study (73 versus 108 previously) and much
greater variation in the physical conditions of the stores. Most notably, there was greater
variation in the basic store plan and layout. We did not have sufficient information about
the remodeling status of each store to include that variable in our analysis, whereas in
the previous study, “Months Since Remodel” had been one of the most powerful
predictors of sales.
The previous set of store sites had higher levels of interior horizontal daylight
illumination, often measured at 100% to 300% of electric illumination levels. The current
study site had lower levels of horizontal daylight illumination, typically measured at 50%
to 100% of electric illumination levels during the 24 month period (100% to 200% during
the 10-month period).
In the previous study we predicted an average 40% net daylight effect for the chain, with
a statistical model that was able to explain about 60% of the variation in the sales data.
In this study we predicted much lower values for an average daylight effect on sales,
from 0% to 6%, with a statistical model that was able to explain about 75% of the
variation in the sales data. However, for individual stores in this study with the most
favorable daylighting conditions (longest hours of daylight, ample parking areas) the
daylighting effect was predicted to be on the order of 40%.
Given the findings of the statistical models for this chain’s average sales, it is likely that
we were near the minimum of a viable study population with only 73 sites, one-third of
which were daylit. A 50% increase in study sites would have increased the certainty of
our findings. Thus, we would recommend that any future study consider 120 sites a
minimum study population, and preferably try to achieve 150 or 200 study sites, of which
about ½ should include the variable of interest.
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RETAIL AND DAYLIGHTING CONCLUSIONS AND DISCUSSION
8.1.1. New Analysis Insights
In this study we were able to include a number of additional analysis methodologies.
First, we were able to compare two different sales periods, which were distinguished by
different illumination levels in the stores. During the period with lower illumination levels,
the relative daylight effect increased by 4.5%, even while average sales for all stores in
the chain rose.
Since we had monthly data, we looked for seasonal effects for daylighting, and in both a
two-dimensional analysis and multivariate analysis, we did not observe any seasonal
differences in sales patterns between daylit and non-daylit stores. We did not have
sufficiently detailed information that would have enabled us to study time-of-day effects.
The implication of the lack of a seasonal effect is that daylight would seem to increase
customer loyalty throughout the year, rather than just during peak daylight periods, like
summer. Thus, we hypothesize that once a customer has decided that they prefer the
retailer’s stores (perhaps partially due to the presence of daylight) then they continue to
shop at the chain throughout the year instead of competitors’ stores. This hypothesis
would seem to be consistent with general marketing theory about brand loyalty.
We also interviewed the store managers and found that a small percentage of them
strongly believed that daylighting increased sales. A survey of store employees found a
significant, but very slight increase in positive attitudes towards the lighting conditions in
daylit stores. We were not able to interview shoppers, as we did in the previous study.
The evidence from the managers’ interviews and employee surveys suggests that
subjective evaluation of daylight conditions is positive, but less robust than the statistical
evidence of an increase in sales.
Discounting subjective evaluation is consistent with other studies that have shown that,
in general, people tend to have little conscious awareness of lighting conditions, and that
they are likely to be influenced in their evaluation of lighting conditions by outside forces
or their education on the issue.1 Perhaps the best evidence we have of this phenomenon
is the increased discussion of the positive effect of daylighting in schools since the
publication of the first PG&E Daylighting in Schools study2. When we began that study,
finding teachers or administrators commenting on the positive effects of daylight was
fairly rare, and required prompting. Now, four years later, when we visit schools for
surveys or observations, teachers and administrators frequently volunteer opinions
about the positive effects of daylight, even when they have no idea why we are visiting
the schools or that we were connected with the earlier study.
8.1.2. Why Daylight Hours is a Better Variable than Daylight Yes/No
The daylight conditions in this chain were far more variable than those in the previous
study site. First, they ranged over more climatic conditions, which could influence
daylight availability. Secondly, there was a greater range in the store designs, with
some stores receiving daylight in only part of their main sales area, and a few stores
receiving daylight from vertical windows rather than horizontal skylights.
1
N Eklund, P Boyce, SN Simpson, “Lighting and Sustained Performance,” Journal of the IESNA, Vol. 29, No
1, Winter 2000, p 116-130.
2
Heschong Mahone Group, Daylighting and Student Performance, PG&E 1999.
51
RETAIL AND DAYLIGHTING CONCLUSIONS AND DISCUSSION
Since we had more information about each store, in this study, we were able to test a
number of different ways of defining the daylight presence in the stores. In addition to
the Daylight indicator variable (yes/no), we tried using indicator variables for Vertical
Glazing (yes/no), and Partial Daylight (yes/no). Ultimately, putting all of the stores on a
single scalar of daylight hours per year seemed to be the most robust approach, and the
most useful in providing design guidance.
The presumption here is that, if the presence of daylight is a positive attribute in a store,
then more hours of daylight per year will provide a greater benefit. As discussed earlier,
in the skylight design typically used in these stores, hours of daylight per year and
intensity of daylight at peak periods are strongly related: i.e. the more intense the
daylight at peak periods, the more hours of useful daylight per year. However, this
relationship is not necessarily a given. It is possible to design skylighting systems that
increase useful daylight during morning and evening hours by increasing the penetration
of low angle sunlight, while reducing peak intensities by reducing the transmission of
high angle sun. While it is our hypothesis that hours of useful daylight per year, rather
than average or peak daylight intensity, will likely be a more useful metric for evaluating
daylighting systems, we cannot distinguish between the two in this analysis.
8.2. Possible Mechanisms for a Daylighting Effect on Sales
The results from the employee survey suggest that the differences in visual quality
between daylit and non-daylit stores can be observed in subjective surveys, but just
barely. Employees’ evaluations showed a subtle difference in lighting quality between
daylit and non-daylit stores. All of the stores, daylit and non-daylit, were judged to be
adequately well lit, and none were judged to have lighting problems. The daylit stores
were judged to have slightly fewer problems, and slightly better lighting quality overall.
Thus, it would seem that the daylight effect is operating at a different level than
conscious perception of the lighting environment. This finding is consistent with the
informal surveys done in the previous study, where over 90%of the shoppers had not
even noticed when the stores had skylights, and yet judged them to be “cleaner” and
“more spacious.”
Lighting designers in general tend to agree that illumination conditions operate below the
level of conscious awareness. Unless the lighting conditions are actively disabling,
either too dark or too glaring, people rarely seem to take conscious note of the lighting
conditions, or the source of the light. In the previous study, we suggested a number of
possible mechanisms for the daylight effect, and below we will discuss these in light of
the current study.
Higher illumination levels is perhaps the most obvious candidate for a causal
mechanism, since the pathway between illuminance levels and performance is well
understood. Other possible benefits of daylight in stores include improved color
rendition, improved lighting quality, greater variability in the store’s appearance by time
of day and season, greater connection to the outdoors through observation of weather
conditions, or improved health and morale of the employees (and possibly shoppers)
due to biological effects of daylight. We will discuss each of these possibilities in turn
Higher Illumination Levels: Higher illumination levels are likely to increase the visibility
of product details, especially for older customers with weaker eyesight. Higher
illumination levels may be responsible for subtle biological effects, as discussed below.
Higher illumination levels may also increase the general perception of the attractiveness
52
RETAIL AND DAYLIGHTING CONCLUSIONS AND DISCUSSION
of the sales space and products, as evidenced in the perceptions of the shoppers
discussed above.
Two findings of this study suggest that higher illumination levels may be partially
responsible for the daylight effect. First of all, more daylight hours per year was directly
associated with greater sales. In skylit stores, more daylight hours per year will also be
directly associated with higher average illumination levels—since as daylight illumination
levels increase there are more hours per year above the design illumination threshold.
Secondly, we found that the relative daylight effect was greater during the 10-month
period when the average electric illumination levels were lower for all stores, thus the
difference in daytime illumination between daylit and non-daylit stores was greater.
During this period, since the electric lighting levels were constant for all stores, the daylit
stores tended to have slightly higher horizontal illumination levels during all daylight
hours.
Illumination levels in the daylit stores is a dynamic condition. Our measurements were
based on hand held illumination readings taken during the late winter months generally
between 10 am and 2 pm. We observed daylit stores to average 66% higher horizontal
illumination than the non-daylit stores during the February-March mid-day period of our
measurements. SkyCalc projections suggest that it would average about 133% higher
during the summer months.
Figure 20 compares the SkyCalc estimated average horizontal illuminance during the
two time periods, 10-month and 24-month, and during peak daylight conditions (July)
and minimum daylight conditions (December) for two average stores in the chain, one
daylit (dashed lines) and one not daylit (solid lines). During the 10-month time period, all
stores operated their electric lights at about ½ power, so that became the minimum
illumination in daylit stores, and the additional daylight contribution always increased
illumination levels during the day. During the 24-month period, all stores operated at full
power, but daylit stores turned off some of their lights during the daytime when threshold
daylight illumination conditions were met.
Hourly Illumination Levels, 10m Hourly Illumination Levels, 24m
Relative Illuminance
Relative Illuminance
Daylit July Daylit July
0 12 0 12
Daylit Dec Daylit Dec
Noon Noon
Non-Daylit Non-Daylit
Figure 20: Hourly Illumination Patterns of Average Daylit and Non-Daylit Stores, 10-
month (left) and 24-month (right) periods
Based on our understanding of the photocontrol operations controlling the electric lights,
and SkyCalc analysis of hourly daylight levels, we believe that during the 24-month
period the daylit stores operated above the non-daylit stores’ illumination level for about
20% of the store hours. Another 40% of the time, they would have been operating at
slightly less than the non-daylit stores’ illumination level, and the remaining 40% of the
time at the same level. Thus, increased horizontal illumination may not be solely
responsible for the increased sales due to a daylight effect.
53
RETAIL AND DAYLIGHTING CONCLUSIONS AND DISCUSSION
Improved Color Rendition: Daylight has the greatest range of spectral wavelengths of
any of our light sources, and is the source our eye has naturally adapted to through
millenniums of evolution. With a continuous spectrum of light, all colors will be more
vivid and have a more naturally rendered appearance under daylight. Any store which
sells products distinguished by color, or where color is a key selection criteria, is likely to
benefit from improved color rendition. This particular retailer, as well as the previous
study participant, does sell some products that would benefit from better color
discrimination. It is possible that there are fewer returns due to poor color selection and
greater attraction to products due to more vivid color rendition. In this study, we did not
collect evidence on this issue.
Greater Depth Perception: There is a second possible effect of the spectrum of
daylight, due to its greater richness in the blue end of the spectrum. One current theory
suggests that people perceive a space to be more brightly lit, and that the resulting size
of their pupil is smaller, under so-called “scotopic” sources of light, those rich in the blue
end of the spectrum. The smaller pupil size is likely to increase the depth of field of the
viewer, allowing a shopper to see greater detail over a wider range of focal distances.
Improved Lighting Quality: The daylit stores had consistently higher illumination levels
on vertical surfaces. During our daytime observations, average vertical illuminance
readings were 62% higher in the daylit stores, and the average minimum readings
(average-standard deviation) were 50% higher. The lowest vertical illumination reading
found in any daylit store was four times higher than the lowest reading in any non-daylit
store.
We examined a set of photographs of a sample of comparable daylit and non-daylit
stores looking for obvious changes in appearance or visual quality that we could not
capture with our surveyors’ light meter readings. The appearance of the product in the
aisles was remarkably similar between the two types of stores. Shadows and highlights
on products appeared very similar.
The most obvious differences between the two store types were in the appearance of
the floor, ceiling and upper walls. The floor in daylit stores was more likely to have
subtle shadows and changes in color, due to alternating patterns of skylight versus
electric light dominating the aisles, whereas the floors in the non-daylit stores were very
uniform in appearance. Similarly the ceiling in the daylit stores might best be described
as “lively,” with daylight bouncing off of many surfaces, compared to a much more
uniform appearance of the ceilings of the non-daylit stores. In the non-daylit stores the
uniform pattern of bright electric lights tended to accentuate the contrasting darkness of
the non-illuminated ceiling. Even when ceilings were the same height, the daylit stores
seemed to have higher ceilings since the ceiling surfaces were more brightly lit,
attracting the viewer’s eye upwards. The daylit stores also tended to have higher
illumination levels on the upper walls and high-mounted signage, creating a bright
horizon and perhaps helping with shopper navigation through the store.
Greater Variability: The range of illumination levels in daylit stores is considerably
greater than in non-daylit stores, and yet the employees judged the daylit stores to have
slightly better lighting quality, and substantially greater lighting uniformity, than the non-
daylit stores. The standard deviation for both vertical and horizontal illumination readings
was four times greater in the daylit versus the non-daylit stores. While uniformity of
illumination is often a key goal of lighting designers, the non-uniformity of the daylit
stores may actually be a positive feature.
54
RETAIL AND DAYLIGHTING CONCLUSIONS AND DISCUSSION
Daylight variability is a function of both space and time. The variation in illumination
levels for daylit stores over time is even greater than the variation in space described
above. Highlighted areas move around the store as the sun moves through the day and
seasons. A daylit store looks different in the morning than in the afternoon, and different
in the summer than in the winter, with slightly changing color of light and patterns of soft
shadows. Such variation in appearance may help to stimulate shopper interest over
repeated visits, and may also help to keep employees alert and mentally engaged over
time.
Connection to the Outdoors: Daylit stores offer some information about the weather
outside of a store. Most of the stores in this chain had diffuse skylights that offered no
view of the sky, but the intensity of daylight or the passing of clouds is clearly
discernable. Rain and hail can be heard through the skylights. This information about
weather conditions outside may also have a stimulating effect, as discussed above
under “Greater Variability.”
Biological Effects: There is a growing body of research being undertaken to
understand the biological effects of light. To date it is clear that bright light exposure
during the day helps reinforce the natural circadian rhythms of various hormones and
neural transmitters. Bright light, most likely in the natural daylight spectrum favoring blue
wavelengths, suppresses melatonin and increases the production of neural transmitters
such as serotonin and dopamine. Healthy circadian rhythms have been implicated in
better immune function and activity patterns. The illumination intensity, duration, timing
and spectral sensitivity of these effects are still being researched, and may likely vary by
time of day. We believe these are fairly slow biochemical effects, unlikely to be
detectable in less than 30 minutes, and most likely to be reinforced over days of
exposure. Thus, short store visits by shoppers are not likely to involve biological effects
from daylight exposure. Store employees, on the other hand, may experience some
health benefits from working in a daylit environment that reinforces their circadian
rhythms. We did not study any indicators that would identify any of these effects. We
hypothesize, however, that positive feelings generated from daylight exposure could
possibly reinforce general positive feelings towards the store for both shopper and
employees.
Although further research will be required to uncover the mechanisms of daylight’s
importance, this study reinforces the finding that daylight does truly increase retail sales.
We have presented evidence from a second retail sector that daylight can increase retail
sales on average for the chain by up to 6%, or for individual stores by up to 40%,
depending on the daylight design, parking lot size, and other store variables. The effect
of daylighting on sales is of a magnitude similar to more traditional retail location
variables and has a value many times greater than the energy savings that accompany
daylighting. These results give us greater confidence in recommending daylighting
strategies for the design of new retail stores.
55
RETAIL AND DAYLIGHTING CONCLUSIONS AND DISCUSSION
56
RETAIL AND DAYLIGHTING APPENDICES
9. APPENDICES
9.1. Retail Survey Forms
Attached are three survey forms that were developed for the study, for on-site data
collection, to interview the manager, and to survey employees. Any potentially
identifying information has been re-coded as option “A,B,C etc”. Some formatting was
lost in transferring the forms, but the basic content and structure remains.
57
RETAIL AND DAYLIGHTING APPENDICES
9.1.1. On-site Survey Form
RETAIL ON-SITE SURVEY
Store location Store number
Exterior site information
Location type urban strip mall big-box rural other Comments
Neighborhood residential commercial industrial
Neighborhood rating dilapidated average high-end Comments
1 2 3 4 5
Building Signage A B C D
Main Street Sign Yes No
Main Street Size Number of lanes Freeway Boulevard Secondary Residential
Main Street Visibility not visible very obstructed some obstructions few obstructions no obstructions
1 2 3 4 5
Co-Tenancy A B C D E
Cloud Condition clear high thin puffy light oc dark oc Comments
Sky Condition stable changeable
Photograph Plan with store number frame #
Main Street toward store frame #
Main Street toward street frame #
100’ from entry toward entry frame #
Others:
Horizontal Illuminance (in parking lot) fc arrival time: AM/PM
Building manager interview
Permission Granted Manager Other
Interview performed Yes No Name
Interior building information
Photos 1 frame #
2 frame #
3 frame #
4 frame #
5 frame #
6 frame #
7 frame #
Notes
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RETAIL AND DAYLIGHTING APPENDICES
Illuminance Measurements
Measurement Time Horiz @ L vert @ L vert @ L vert @ R vert @ R vert @ R vert @
4 feet
location 2 feet 4 feet 6 feet 2 feet 4 feet 6 feet
Wide aisle u/s
Wide aisle b/s
Narrow aisle u/s
Narrow aisle b/s
Back aisle u/s
Back aisle b/s
Checkout 1 2 3
n/a n/a n/a n/a
Legend: u/s=under skylights or if no skylight exists, b/s = between skylights
Reflectances
Floor A B Other
Upper Walls A B Other
Ceiling A B Other
Structure A B Other
Cleanliness very cln. clean normal dirty very dirty
1 2 3 4 5 Comment
Luminaires
Verify Verify mounting height and spacing with plan survey.
Layout A B C D Other
Flicker Yes No Hum Yes No
Cleaning Clean Dirty
Controls A B C D Other
Skylights no skylights (if no skylights, ignore the following 5 questions)
Type A B Other
Well Depth A B C D E Other
Cleaning Clean Dirty Discoloring Yes No
Obstructions A B C D Other
Thermal Environment
Heating A B Dry bulb temp. °C or °F
Air movement A B Air velocity ft/min or m/s
Cooling A B Cycling Yes No
Aural Environment silent quiet normal loud very loud
Ambient 1 2 3 4 5 Decibels
PA 1 2 3 4 5 Decibels
Air 1 2 3 4 5 Decibels
Primary Noise Source HVAC Street Chckout Patrons Other Location
Olfactory Environment very pleasant neutral very unpleasant
Pleasantness 1 2 3 4 5 Describe
Horizontal Illuminance (in parking lot) fc departure time: AM/PM
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9.1.2. Manager Interview
Building manager interview (confidential)
Interviewee Name Time
Title Manager Other
Gender Male Female Eye Glasses Yes No
Age <30 31-40 41-50 51-60 > 61
Years at this store Years with this company
1. Have you ever worked at a different store location for this company? Yes No
If Yes, location dates from to
2. What do you think are this store’s major advantages compared with similar stores?
Neighborhood Visibility Personnel Maintenance
Other
3. Have any new competitors opened up nearby (X distance +/-) in the past three years?
Other
4. Have any local events in the past three years dramatically affected sales? Yes No
Construction Resurfacing Flood Personnel Crime
Other
Date(s) approx. duration
5. On a scale of 1 to 5, how do you rate the electric lighting system in this store?
Part on: 1 Poor 2 Fair 3 Adequate 4 Good 5 Superior
All on: 1 Poor 2 Fair 3 Adequate 4 Good 5 Superior
Comments
6. Do you think the electric lighting system in this store impacts sales performance?
Part on: 1 very negative 2 negative 3 no effect 4 positive 5 very positive
All on: 1 very negative 2 negative 3 no effect 4 positive 5 very positive
Comments
7. Have you had any problems with the electric lighting system? Yes No
frequent burn out flicker hum switching problems
Other
8. Did this store experience any unexpected loss of power last year? Yes No
If Yes, did customers have to leave the store? Yes No
Date(s) approx. duration
Comments
If there are no skylights, skip the following questions and check here no skylights
9. On a scale of 1 to 5, how do you rate the light from the skylights in this store?
1 Poor 2 Fair 3 Adequate 4 Good 5 Superior
Comments
10. Do you think the skylights in this store impact sales performance?
1 very negative 2 negative 3 no effect 4 positive 5 very positive
Comments
11. Have you had any problems with the skylights? Yes No
Water leaks Breakage Crime Fall-through
Other
Date(s) approx. duration
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9.1.3. Employee Survey
Store location Store number
LIGHTING QUALITY SURVEY
You are being asked to participate in a voluntary study about the lighting quality in this store. This study is
conducted by an independent consultant and your answers are confidential. We ask that you complete this
survey and return it to the designated person as soon as possible. If you would prefer, you can mail the form
to us directly at “Lighting Survey”, 11626 Fair Oaks Blvd. #302, Fair Oaks, CA 95628. Thank you!
Do you work full or part time? full time part time
Where in the store do you primarily work? sales floor back office other
Please indicate your age. <30 31-40 41-50 51-60 > 61
How long have you worked at this store? In years or fraction of a year
Please answer the following questions related to your experience of the lighting in this store as it is today.
For each question, please circle the number that most closely matches your opinion, where 1 means you
strongly disagree and 7 means you strongly agree.
strongly somewhat no somewhat strongly
disagree disagree disagree opinion agree agree agree
a) Overall, the lighting quality in this store is comfortable.
1 2 3 4 5 6 7
b) The lighting helps make the merchandise look appealing.
1 2 3 4 5 6 7
c) The store is uncomfortably bright.
1 2 3 4 5 6 7
d) The store is uncomfortably dim.
1 2 3 4 5 6 7
e) The light fixtures themselves are too bright.
1 2 3 4 5 6 7
f) There is too much light in some areas and not enough in others.
1 2 3 4 5 6 7
g) The lighting makes it difficult to examine detail closely.
1 2 3 4 5 6 7
h) Reflections on the merchandise are sometimes a problem.
1 2 3 4 5 6 7
i) Skin tones look unnatural under this lighting.
1 2 3 4 5 6 7
j) It is difficult to distinguish shades of color under this lighting.
1 2 3 4 5 6 7
k) The lights sometimes flicker or hum annoyingly.
1 2 3 4 5 6 7
For question 12, please circle the number that most closely matches your opinion of the lighting in this store
compared to other stores with which you are familiar.
l) How does the lighting in this store compare to lighting in similar stores?
worse the same better
1 2 3 4 5 6 7
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Retail Sales Model Results
9.1.4. Natural log Models
Summary Stats for Natural Log Models
Variable Description Variable Min Max Range Ave SD
ln(sales index 24m) LogSales10m 7.09 8.56 1.48 7.78 0.32
ln(sales index 10m) LogSales24m 6.97 8.53 1.56 7.72 0.36
ln(Total Area) logArea 1.00 1.06 0.06 1.04 0.01
Longer work week, yes/no Hours 0.00 1.00 1.00 0.36 0.48
ln(Age) logAge 1.00 3.68 2.68 2.11 0.59
Percent Population Growth, 2000-1990 PopGrow 1.00 13.18 12.18 5.09 2.55
Number of sister stores within certain radius Co-mktg 1.00 5.00 4.00 3.93 1.26
Number of competitor stores within radius 1 Compet 1 0.00 10.00 10.00 2.40 2.22
Storefront height scalar Height 1.00 3.57 2.57 1.91 0.47
ln(Parking) logPark 1.00 1.29 0.29 1.17 0.07
Outlier Store Out44
Daylit hours per year greater than threshold DayHrs 270.00 1800.32 1530.32 1090.55 408.86
Parking * DayHrs ParkDH 447.95 4557.73 4109.77 2654.69 1289.44
Figure 21: Summary Statistics for Natural Log Models
These are only the variables found significant in the Log Models. For more information
on other variables considered, look at the Descriptive Statistics table for the Linear
Models, also included in the Appendix. For all models, we have dropped out the
intercept values for the model equations, since they do not effect any results, and they
became difficult to keep consistent across the transformed linear and log models.
Model Name: LN 10m
Variable Description Variable B Std. Error t Sig.
ln(Total Area) logArea 0.59 0.18 3.26 0.002
ln(Age) logAge 0.28 0.05 6.10 0.000
Transportation variable, 1990 Transport 0.00 0.00 -2.53 0.014
Education variable 1990 Education 0.00 0.00 2.87 0.006
Number of sister stores within certain radius Co-mktg 0.07 0.02 3.38 0.001
Number of competitor stores within radius 1 Compet 1 -0.05 0.02 -2.81 0.007
Storefront height scalar Height -0.01 0.00 -2.27 0.027
ln(Parking) logPark -0.41 0.08 -4.94 0.000
Outlier Store out440 0.68 0.18 3.84 0.000
Daylit hours per year greater than threshold DayHrs 0.00 0.00 -2.50 0.015
Age * DayHrs AgeDH 0.00 0.00 -1.71 0.092
Parking * DayHrs ParkDH 0.00 0.00 3.63 0.001
Model Summary:
RMSE 0.17
R^2 75.7%
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Figure 22: Log Model of 10-Month Sales, 2001
Model Name: LN 99, 01
Variable Description Variable B Std. Error t Sig.
ln(Total Area) logArea 7.694 2.08 3.69 0.001
ln(Age) logAge 0.246 0.05 5.19 0.000
Transportation variable, 1990 Transport -0.00002 0.00 -3.84 0.000
Education variable 1990 Education 0.00001 0.00 3.68 0.001
Number of sister stores within certain radius Co-mktg 0.091 0.02 3.85 0.000
Number of competitor stores within radius 1 Compet 1 -0.056 0.02 -2.97 0.004
Storefront height scalar Height -0.161 0.07 -2.34 0.023
ln(Parking) logPark -1.823 0.41 -4.41 0.000
Outlier Store out440 0.651 0.20 3.27 0.002
Daylit hours per year greater than threshold DayHrs -0.001 0.00 -3.06 0.003
Parking * DayHrs ParkDH 0.00024 0.00 3.20 0.002
Model Summary:
RMSE 0.19
R^2 74.7%
Figure 23: Log Model of 24-Month Sales, 1999-2000
Model Name: LN 01
Variable Description Variable Order of Entry Partial r^2
ln(Total Area) logArea 2 0.069
ln(Age) logAge 1 0.379
Transportation variable, 1990 Transport 12 0.026
Education variable 1990 Education 11 0.009
Number of sister stores within certain radius Co-mktg 5 0.038
Number of competitor stores within radius 1 Compet 1 6 0.037
Storefront height scalar Height 7 0.023
ln(Parking) logPark 4 0.050
Outlier Store out440 3 0.059
Daylit hours per year greater than threshold DayHrs 9 0.038
Age * DayHrs AgeDH 10 0.016
Parking * DayHrs ParkDH 8 0.013
Figure 24: Order of Entry and Partial R2, Log 10 Month Sales, 2001
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RETAIL AND DAYLIGHTING APPENDICES
Model Name: LN 99-00
Variable Description Variable Order of Entry Partial r^2
ln(Total Area) logArea 2 0.077
ln(Age) logAge 1 0.340
Transportation variable, 1990 Transport 8 0.015
Education variable 1990 Education 9 0.055
Number of sister stores within certain radius Co-mktg 6 0.037
Number of competitor stores within radius 1 Compet 1 3 0.055
Storefront height scalar Height 7 0.041
ln(Parking) logPark 5 0.041
Outlier Store out440 4 0.045
Daylit hours per year greater than threshold DayHrs 11 0.039
Parking * DayHrs ParkDH 10 0.004
Figure 25: Order of Entry and Partial R2, Log 24 Month Sales, 1999-2000
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9.1.5. Linear Models
Summary Stats for Linear Models (all variables considered)
Variable Description Variable Name MIN MAX RANGE MEAN STD
OUTCOME (DEPENDANT) VARIABLES
Sales index per store, 24 mo avg for 1999-2000 Sales24 1068.40 5068.17 3999.77 2390.61 867.77
Sales index per store, 10 mo avg for 2001 Sales10 1195.07 5234.18 4039.11 2515.70 828.65
EXPLANATORY (INDEPENDANT) VARIABLES
CORPORATE VARIABLES
Total Sales Area Scalar Area 1.00 1.87 0.87 1.50 0.19
Longer work week, yes/no Hours 0.00 1.00 1.00 0.36 0.48
Store Age Scalar, relative age from date of first opening Age 1.00 19.00 18.00 4.17 3.03
Manager seniority scalar Mgr 1.00 56.00 55.00 21.64 13.65
CENSUS VARIABLES
Housing Status Housing 2182.00 45229.00 43047.00 22983.55 11123.58
Population Density, 2000 Pop 6701.00 321692.00 314991.00 86522.51 60056.14
Percent Population Growth, 2000-1990 PopGrow 1.00 13.18 12.18 5.09 2.55
Ethnic Status, 2000 Ethnic 0.29 0.92 0.64 0.63 0.15
Households, 2000 Household 4026.00 175163.00 171137.00 44657.60 32533.27
Income 1990 Income 10813.34 29831.75 19018.41 17537.69 4711.22
Economic Status 1990 Econ 0.02 0.23 0.21 0.10 0.05
Education variable 1990 Education 2886.00 143727.00 140841.00 42678.78 31059.53
Language variable, 1990 Language 44.00 97023.00 96979.00 8873.19 15428.92
Transportation variable, 1990 Transport 286.00 36884.00 36598.00 7879.32 6884.83
LOCAL MARKET INFLUENCES
Number of sister stores within certain radius Co-mktg 1.00 5.00 4.00 3.93 1.26
Number of competitor stores within radius 1 Compet 1 0.00 5.00 5.00 1.44 1.29
Number of competitor stores within radius 2 Compet 2 0.00 10.00 10.00 2.40 2.22
Co-tenant scalar Cotenant 0.00 4.00 4.00 1.49 1.39
Number of lanes on the main street Lanes 2.00 8.00 6.00 4.55 1.32
Street visibility scalar Visible 1.00 5.00 4.00 3.30 1.14
Building signage is "typical" yes/no Sign 0.00 1.00 1.00 0.90 0.30
Flag for negative sales event in neighborhood Event 0.00 1.00 1.00 0.25 0.43
Storefront length scalar Length 1.00 3.79 2.79 1.97 0.57
Storefront height scalar Height 1.00 3.57 2.57 1.91 0.47
Parking scalar Parking 1.00 3.63 2.63 2.22 0.64
STORE COMFORT CONDITIONS
Daylit hours per year greater than threshold DayHrs 270.00 1800.32 1530.32 1090.55 408.86
Average of all vertical illuminace readings, scalar VertAve 2.21 31.83 29.62 6.93 4.02
Standard Deviation of vertical illuminace scalar VertSD 1.00 41.13 40.13 2.99 4.67
Atypical luminaire layout yes/no Luminaire 0.00 1.00 1.00 0.12 0.33
Electric lighting percent on, scalar Lightson 1.00 4.00 3.00 2.44 0.78
Ceiling height scalar ClgHt 1.00 2.50 1.50 1.50 0.32
Noticeable air movement ,yes/no Air 0.00 1.00 1.00 0.10 0.30
Odor scalar Smell 2.00 5.00 3.00 3.04 0.48
Noise scalar Noise 3.00 8.00 5.00 5.19 1.24
INTERACTION VARIABLES (all based on scalars above)
Sales Area * DayHrs AreaDH 1.00 6.92 5.92 4.66 1.76
Age * DayHrs AgeDH 1.00 17.79 16.79 5.73 4.13
Longer Hours * DayHrs HoursDH 1.00 3.83 2.83 2.71 1.19
PopGrowth * DayHrs PopGrowDH 1.00 13.18 12.18 5.09 2.55
No. sister stores * DayHrs MktgDH 1.00 16.67 15.67 7.88 3.69
No. Competitors w/in Radius 1 * DayHrs Comp1DH 1.00 21.27 20.27 6.96 5.19
Frontage height * DayHrs HeightDH 1.00 8.38 7.38 4.70 2.10
Parking * DayHrs ParkDH 1.00 10.17 9.17 5.93 2.88
Area*DayHrs*Hours AreaDHhours 1.00 4.03 3.03 2.79 1.25
Figure 26: Summary Statistics for All Variables Considered in Linear Models
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Model Name: Linear 01
Variable Description Variable B Std. Error t Sig.
Total Sales Area Scalar Area 1051.87 327.90 3.21 0.002
Store Age Scalar, relative age from date of first openingAge 146.97 24.53 5.99 0.000
Transportation variable, 1990 Transport -0.04 0.01 -2.67 0.010
Education variable 1990 Education 0.01 0.00 2.78 0.007
Number of sister stores within certain radius Co-mktg 180.89 52.71 3.43 0.001
Number of competitor stores within radius 1 Compet 1 -122.49 42.79 -2.86 0.006
Storefront height scalar Height -416.25 150.17 -2.77 0.007
Parking scalar Parking -579.25 105.31 -5.50 0.000
Outlier Store Out44 2183.45 449.62 4.86 0.000
Daylit hours per year greater than threshold DayHrs50 -1.41 0.43 -3.25 0.002
Age * DayHrs AgeDH -0.08 0.05 -1.73 0.089
Parking * DayHrs ParkDH 0.73 0.18 4.22 0.000
Model Summary:
RMSE 439.95
R^2 76.5%
Figure 27: Linear Model of 10 Month Sales, 2001
Model Name: Linear 99-00
Variable Description Variable B Std. Error t Sig.
Total Sales Area Scalar Area 1305.09 340.88 3.84 0.000
Store Age Scalar, relative age from date of first o Age 110.68 22.56 4.91 0.000
Transportation variable, 1990 Transport -0.06 0.02 -4.14 0.000
Education variable 1990 Education 0.01 0.00 3.95 0.000
Number of sister stores within certain radius Co-mktg 217.31 56.38 3.85 0.000
Number of competitor stores within radius 1 Compet 1 -129.86 45.18 -2.87 0.006
Storefront height scalar Height -388.92 161.60 -2.41 0.019
Parking scalar Parking -516.95 111.11 -5.61 0.000
Outlier Store Out44 1981.51 484.82 4.09 0.000
Daylit hours per year greater than threshold DayHrs -1.57 0.45 -3.47 0.001
Parking * DayHrs ParkDH 0.64 0.18 3.47 0.001
Model Summary:
RMSE 474.65
R^2 75.3%
Figure 28: Linear Model of 24 Month Sales, 1999-2000
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RETAIL AND DAYLIGHTING APPENDICES
Model Name: Linear 01
Variable Description Variable Order of Entry Partial r^2
Total Sales Area Scalar Area 4 0.054
Store Age Scalar, relative age from dat Age 1 0.318
Transportation variable, 1990 Transport 12 0.028
Education variable 1990 Education 11 0.006
Number of sister stores within certain rCo-mktg 6 0.034
Number of competitor stores within radCompet 1 5 0.029
Storefront height scalar Height 7 0.035
Parking scalar Parking 3 0.079
Outlier Store out440 2 0.104
Daylit hours per year greater than thDayHrs50 9 0.053
Age * DayHrs AgeDH 10 0.017
Parking * DayHrs ParkDH 8 0.008
Figure 29: Order of Entry and Partial R2, Linear 10 Month Sales, 2001
Model Name: Linear 99-00
Variable Description Variable Order of Entry Partial r^2
Total Sales Area Scalar Area 4 0.056
Store Age Scalar, relative age from date of first opAge 1 0.276
Transportation variable, 1990 Transport 8 0.016
Education variable 1990 Education 9 0.060
Number of sister stores within certain radius Co-mktg 6 0.033
Number of competitor stores within radius 1 Compet 1 5 0.044
Storefront height scalar Height 7 0.052
Parking scalar Parking 3 0.070
Outlier Store Out44 2 0.088
Daylit hours per year greater than threshold DayHrs50 11 0.050
Parking * DayHrs ParkDH 10 0.001
Figure 30: Order of Entry and Partial R2, Linear 24 Month Sales, 1999-2000
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9.1.6. Linear Transaction Models
Summary statistics for the transaction index models are the same as the linear sales
index models, and are presented earlier in Figure 26.
Model Name: Linear Transactions 2001
Variable Description Variable B Std. Error t Sig.
Total Sales Area Scalar Area 16.23 6.49 2.79 0.007
Store Age Scalar, relative age from date of first o Age 2.68 0.48 5.60 0.000
Housing variable, 2000 Housing 0.0004 0.00 3.37 0.001
Number of sister stores within certain radius Co-mktg 2.80 1.05 2.65 0.010
Number of competitor stores within radius 1 Compet 1 -3.87 0.83 -4.65 0.000
Storefront height scalar Height -9.35 2.80 -3.34 0.001
Parking scalar Parking -11.77 2.04 -5.77 0.000
Outlier Store Out44 36.92 8.75 4.22 0.000
Daylit hours per year greater than threshold DayHrs50 -0.0255 0.01 -3.07 0.003
Age * DayHrs AgeDH -0.0018 0.00 -1.96 0.054
Parking * DayHrs ParkDH 0.0131 0.00 3.95 0.000
Model Summary:
RMSE 8.44
R^2 77.2%
Figure 31: Linear Model of 10 Month Transactions, 2001
Model Name: Linear Transactions 9900
Variable Description Variable B Std. Error t Sig.
Total Sales Area Scalar Area 16.23 6.49 2.66 0.010
Store Age Scalar, relative age from date of first o Age 2.51 0.46 5.48 0.000
Transportation variable, 1990 Transport -0.0008 0.00 -2.98 0.004
Percent Population Growth, 2000-1990 PopGrow -16.5897 5.86 -2.83 0.006
Housing variable, 2000 Housing 0.0005 0.00 3.08 0.003
Number of sister stores within certain radius Co-mktg 3.61 1.16 3.10 0.003
Number of competitor stores within radius 1 Compet 1 -3.95 0.91 -4.33 0.000
Storefront height scalar Height -7.40 3.31 -2.24 0.029
Parking scalar Parking -10.87 2.24 -4.84 0.000
Outlier Store Out44 25.76 10.23 2.52 0.015
Daylit hours per year greater than threshold DayHrs -0.04 0.01 -3.88 0.000
Parking * DayHrs ParkDH 0.01 0.000 3.87 0.000
Model Summary:
RMSE 9.51
R^2 75.2%
Figure 32: Linear Model of 24 Month Transactions, 1999-2000
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9.2. Parking Area Verification Process
During the course of analysis it was discovered that some of the parking lot counts
collected in the initial plan review phase of data collection did not seem plausible. Many
of the site plans reviewed were old or incomplete, and it was possible that the parking lot
had been modified since the plan date. Since the parking lot variable was quite
significant in initial models of sales performance, we decided to verify the parking lot
counts during the study period.
We obtained parking lot counts from the retailer for about 80% of the store sites.
However, these counts were of uncertain dates and based on a variety of counting
methodologies. We also obtained low-resolution aerial photographs for about 80% of
the sites (not the same 80%), from which we could estimate the parking capacity of the
lots. While the aerial photos were considered the most reliable in terms of time period
(they were all from approximately the study period) they were often difficult to interpret.
We followed the following methodology to finalized the parking data:
1) We first compared the retailer provided counts to our initial counts.
2) If the two counts were within 15% of each other, we assumed our count to be
accurate, since it was based on a consistent counting methodology.
3) If the counts varied by more than 15%, we proceeded to examine the aerial
photographs to see if we could determine which count was (more) correct. Using the
aerial photograph counts, we also attempted to validate any one of three possible
methodologies that might have been used to generate the retailer counts. From this
exercise, it was determined that:
a) The retailer counts did not use a consistent counting methodology, as we had
been warned
b) There was not a clear trend between the retailer or HMG counts being more
accurate or consistent
c) We also compared aerial counts to a few stores where we could verify the actual
parking counts with site visits. In these cases, we found the aerial counts to be
within 5-15% of the actual counts.
4) Therefore, if the aerial and one of the other counts were within 15% of each other,
we accepted either the retail or HMG count that was closest to the aerial count.
5) If neither the retail or HMG count were within 15% of the aerial count, we accepted
the aerial count.
6) There were a few cases where we could not validate the HMG counts via this
method (because not all three sources were available), in which case we accepted
the HMG count.
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9.3. Statistical Terminology
The following briefly describes key statistical terms in the report.
Term Name Definition
R Correlation Coefficient Measures the strength of the linear relationship
between two variables
Or
It can take on the values from -1.0 to 1.0, where -1.0
Pearson correlation is a perfect negative (inverse) correlation, 0.0 is no
correlation, and 1.0 is a perfect positive correlation.
p p-value A p-value is a measure of the certainty you have that
a relationship exists between an explanatory variable
Or
(e.g., smoking) and an outcome variable (e.g.,
significance cancer). It is a measure of how much evidence you
have that the null hypothesis – that no relationship
Or exists – is not true. The p-value is the probability that
you are falsely rejecting the null hypothesis, i.e., that
Sig. you are falsely declaring that a relationship exists
(e.g., between smoking and cancer.)
The smaller the p-value, the more evidence you
have. The probability of a false rejection of the null
hypothesis in a statistical test is called the
significance level. A p-value can vary from >.00 to
<1.0. The significance level is 1-p, expressed as a
percentage. So if a p-value is .01, the significance
level is 99%.
Typically, in statistical tests, one sets a threshold for
an acceptable significance level. In such a case, if
the p-value is less than some threshold (usually .05,
sometimes a bit larger like 0.1 or a bit smaller like
.01) then you reject the null hypothesis, and conclude
that there is a reasonable likelihood of a relationship
between the explanatory variable and the outcome.
F-Test A statistical hypothesis test based on the F
distribution where the null hypothesis is that a set of
B coefficients are simultaneously zero. The
alternative hypothesis is that there is at least one B
coefficient in the set that is not zero.
Figure 33: Glossary of Statistical Terminology
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Term Name Definition
R2 Regression correlation A value between 0 – 1.0 that indicates how well an X
coefficient value (or the independent or explanatory variables in
the regression) explains a Y value (the dependent
variable). Technically, the regression equation is: Y=
B0+B1X1+ B2X2+…+ BnXn+e
where B0= intercept, e=error,
so as Xs change, Y, the dependent variable, also
changes., and variations in X values cause variations
in Y.
R2 is defined as the percentage of total variation in Y
explained by the independent variables.
If R2 is equal to 1, then entire variation in Y is
explained by the independent variables, i.e. the
model is very good, and the X variables have perfect
explanatory power (for explaining Y). So, the higher
the value of R2, the better the model is for that set of
data. Models explaining data that have a high
degree of inherent variation, such as individual
behavior, will have a much lower R2 than models
explaining more predictable events, such as group
averages.
B B Coefficient Technically, the regression equation is:
Y= B0+B1X1+ B2X2+…+ BnXn+e
where B0 is the intercept (constant), and
B1 ,B2 ,…,Bn are the slopes of the regression
equation, or the coefficients of the Xs, (or the
independent variables), and e is error.
A particular Bi (i=1,2,…,n) shows how a particular Xi
variable is related to Y. If a Bi coefficient is a positive
number, an increase in Xi by one unit increases Y by
the amount of the Bi coefficient.
df Degrees of Freedom The total number of observations minus the number
of restrictions on the observations. For a regression
model, the degrees of freedom is equal to the
(number of observations - one) – (number of
explanatory variables in the model). For example,
the log models in this report consist of (73-1)-(11) =
72-11=61 degrees of freedom.
72